This tragic story has four protagonists - at least, to start with, of equal prominence - Sigmund Freud (aged 26), playing the doctor, Ignaz Schönberg (also 26), as the patient, and two sisters: Martha Bernays (aged 21) and her, four years younger, sister Minna (aged 17).
Ignaz Schönberg's fateful relationship to Freud is closely related to Freud's courtship of Martha, as well as to Ignaz' contemporary courtship of Minna. As soon as Freud laid his eyes on the Bernays sisters in April 1882, even though he was to be soon to be engaged to the elder sister, he found both of them extremely attractive.
Freud’s choice of whom to court was limited since, Minna was engaged to a friend of Freud's, Schönberg. (1)  Her engagement took place, on 12 February 1882, at the age of 17.  (2)
Already, the fact that Freud considered Ignaz his friend didn’t bode good for the former. Being Freud's friend was always a risky business, since, as he explained in his dream book, due to the bizarre workings of his sick mind, he always turned a friend into an enemy whom he wanted to get permanently rid of.

Freud's friends and enemies

As Freud explained:
an intimate friend and a hated enemy have always been indispensable to my emotional life…not infrequently… friend and enemy have coincided in the same person. (3)
Thus, without realising it - competing with Freud for Minna’s heart and more carnal favours – Schönberg was doomed.
There’s no doubt about Freud’s jealousy of his former "friend's", Ignaz' closeness to Minna. No doubt, Freud believed that he had the “right”, not only to Martha but, also to her younger sister.

Freud's interest in us

On one occasion, when walking with Martha, Freud was asking her incessantly about Minna. When Martha told Minna about it, the younger sister replied, It is very kind of Herr Doctor to take so much interest in us. (4) Without a doubt, she was well aware of Freud's obsessive interest in her and possibly flattered by it. 
As one biographer observed, Almost from the moment Freud took a passionate interest in Martha Bemays in April 1882, he was drawn to her younger sister Minna …. He wrote her intimate, amorous letters. (5)
Thus, besides Martha, Freud also was romancing, or rather grooming, Minna - still a teenager - behind her fiancé's back.
(1) Schur, Max, Freud: Living and Dying, (1972, p. 34).
(2) Steiner, Ricardo, Die Brautbriefe: The Freud and Martha correspondence, Int J Psychoanal (2013) 94:863–935, p. 884.
(3)  SE 5, p. 483.
(4) Jones, Ernest, Sigmund Freud: Life and Work: The Formative Years and the Great Discoveries, (1953, p. 108).
(5) Gay, Peter, Reading Freud: Explorations and Entertainments, (1990, p. 165).     

Even though he was engaged to her older sister, in his letter of August 22, 1882, Freud confessed to Minna:
I have to turn to you, because in every respect you are the closest to me. (1).
Already from the start, Martha was coming second.
            Besides the sexual innuendoes in the letters to both sisters, what is more disturbing is the daring with which Freud ignored the fact that Minna was already engaged and the way he isolated his "friend" Schoenberg from his fiancé.
Notably, in the letter of November 2, 1882, to both sisters - his future harem - Freud addressed them as, My beloved Minna, my dear Martchen. There’s no doubt, which of the two Freud preferred. Closing the letter, Freud wrote,
Leaving you with a hearty greeting for the one and more, more than space allows for the other. (2)
            There can be no doubt about Freud’s passion for Minna. 

The Freudian triangle or a ménage à trois

In fact, In these early letters the difference between the clearly sexual casting of Martha and the vaguer libidinous casting of Minna is very evident. In fact, for years Freud had confined her within the triangle she so fittingly describes as “Martha and you and l. (3)
And, the only obstacle to Freud's conquest of Minna was her fiancé.
An important part in Freud's seduction of the young woman was his efforts at isolating her from her fiancé. Thus, planning to visit the Bernays family, on August 24, 1884, writing to Minna, rather than to Martha, Freud told her that, we, thus he and Martha, will often take you with us and be very nice to you. (4)
(There's no explanation as to what this niceness, on Freud's part, was supposed to entail.) Without doubt, though, this was an obvious invitation for Minna to become a part of the premarital ménage à trois, totally excluding her fiancé.

Wild and passionate

Over time, Freud and Minna became very good "friends", frequently writing to each other. Cunningly, an apparent deception, Freud claimed - that, since both of them, were wild, passionate people (5)  - they were unsuitable for one another.
          Chronology is important for the progress of events leading to Schönberg's death. As Freud recounted many years later in his, An Autobiographical Study, (due to unsatisfactory performance), in the year 1882, he was forced by professor Brucke to leave the Institute where until then he was employed.
Freud's place was instead given to yet another of Freud's “friend”, Josef Paneth. Notably, around the same time, thus in 1882, both of Freud's friends, Schönberg and Paneth, contracted "tuberculosis" that subsequently resulted in their deaths.

Alleged tuberculosis

 As Freud's doctor, Schur, recounted,
Martha's sister Minna was engaged to a friend of Freud's, Schönberg, who contracted tuberculosis. And, in a footnote, notably, Schur pointed out that,
So apparently did Freud’s friend and benefactor, Paneth. (6)
Remarkably, not only Schur makes a connection between Schönberg’s and Paneth’s illness, but he also calls into doubt the latter's diagnosis. Could Paneth's lethal illness be a result of a deliberate poisoning of him by the Viennese Lucretia Borgia, aka Freud, rather than of a coincidental TB infection? Could also Schönberg's illness have the same origin?
Notably, while Freud competed with Schönberg for Minna’s love, Paneth inherited Freud’s position as a junior researcher at the Brucke’s institute, thus, both of them becoming Freud's enemies.
There's no doubt that regarding both of his "friends", Freud not only had a motive but also, as a doctor, had the means. And there’s an obvious link between the two of them.
Both Schönberg and Paneth, were Freud’s “friends”, as well as competitors, and both died of the same tuberculoid disease.

Schönberg's lung disease

The information below, about Freud's exchange of letters with Ignaz Schönberg's fiancé, Minna comes from the French publication, Correspondence 1892-1938: Freud Sigmund; Bernays Minna, (2015).
Writing to Freud, on January 2, 1883, Minna called him respected doctor. Little did she know that, besides wearing a doctor's hat, Freud had nothing in common with medicine. Of course, she could not have known about Freud being a medical impostor, as a fateful result, in good faith, accepting Freud as Schönberg's doctor. This naivety applied to Ignaz's family, as well.
In the letter of January 22, 1983 - assuming the dating is correct - Freud referred to the relapse of Schönberg's illness. As the editor explained, It is not clear whether this was tuberculosis, a disease from which, allegedly, Schönberg died. It appears that this relapse has nothing to do with Schönberg's incurable disease that followed.
Writing to Minna, three months later, on May 19, 1883, Freud informed her that Schönberg caught a lung disease. He also admonished her not to visit her fiancé again. (7) Freud didn't state that Schönberg's disease was diagnosed as tuberculosis, only informed Minna that professor Nothnagel, who visited Ignaz forbade any visits. Whether Nothnagel did visit Ignaz is doubtful.  Professors, as a rule, don't do home visits to patients. Probably, this was Freud's subterfuge presented to Minna, so that she wouldn't visit her fiancé.
N.B. In the beginning of September 1883, Nathan Weiss, poisoned by Freud with a lethal pathogenic, committed suicide. Was Ignaz's lung disease caused by the same pathogen?
(1)  Crews, Frederick, The Making of an Illusion, (2017, p. 565).
(2) Maciejewski, Franz, Minna Bernays as "Mrs. Freud": What Sort of Relationship Did Sigmund Freud Have with His Sister-in-Law? American Imago, Vol. 65, No. 1, Genuinely New Light on Freud (Spring 2008, p.8).
(3) Maciejewski, 2008, p.12).
(4) Freud, Sigmund, The Letters of Sigmund Freud, 1853-1939, (1960, p. 125).
(5) Jones, Ernest, Sigmund Freud: Life and Work: The Formative Years and the Great Discoveries, 1856-1900.  (1953, p. 164).
(7) Schur, Max, Freud: Living and Dying, (1972, p. 34).
(8) Freud, Sigmund & Bernays, Minna,  Correspondence 1892-1938: Freud Sigmund; Bernays Minna, (2015).

According to Schur, Tuberculosis was very frequent in Vienna. Even at the time I was a student it was called [in Latin] "Morbus Vindobonensis" [the disease of Vienna]. It was the dreaded disease. (1)
Oddly, Schönberg’s alleged TB wasn’t dreaded by neither Freud, nor the Bernays sisters who frequently met with Schönberg. Even more oddly, neither of them contracted the disease, only somehow Schönberg did. Also, in Paneth’s case, neither his family nor, his “friend”, Freud was infected by his alleged TB. So, maybe, in both cases, it wasn’t the TB, but some other deadly disease, that both Schönberg and Paneth were infected with.

Second examination

This is what, Freud, the doctor in the making, on the Sunday, July 29, 1883, reported to Minna:
I examined him and was able to note a marked improvement compared with the first examination, a few days after he fell ill, but I refuse ... to consider him to be in good health.
Can we trust Freud's diagnosis, considering that Freud was, clumsy in the use of the instruments when doing the practical work in the Policlinic? * (2) Hardly. In fact, It is doubtful he knew how to use a stethoscope. Notably,  the examination took place on a Sunday, thus in a private home.
In the same letter, Freud showed his true hateful colours, writing: I feel more and more hatred now [toward Minna's mother] that I do not see her in person. Minna's reaction to Freud's confession isn't known.

Away for the winter

In The Letters of Sigmund Freud, the first mention of Schönberg's illness appeared in Freud’s letter to Martha, of September 8, 1883, in which, without mentioning the TB, Freud claimed that he was trying, to get Schönberg away for the winter. (3)

Satisfied with his condition

Apparently, at that date, Schönberg was already seriously ill. Notably, there's no mention of any other doctor, than Freud, being involved in Ignaz's treatment. Oddly, already on the next day, Freud informed Martha,
that Schönberg comes to see me every day, [and] that I am fairly satisfied with his condition. (4)
Thus, Freud, playing Ignaz's doctor, was treating Ignaz for his illness.

And, on August 6, 1885, informing Martha about Schönberg’s condition, Freud wrote,
As you have seen him [Schönberg] recently I won't describe to you what he looks like now [but he did!]- without blood or flesh, without voice or breath. One of his lungs is completely destroyed, and the other probably riddled with disease. I consider him a lost man. Notably, unlik Schur, Freud didn’t mention the “affected” larynx.
(N.B. It is a good question how Freud could have known about the damage to Schönberg’s lungs, since the x-ray use, for this purpose, began first in 1896. Remarkably, Freud revealed to Martha that Schönberg allegedly, suddenly realized that … his love had died before him. This sounds like an intentional falsehood intended to distance Minna from her fiancé. And since, no doubt, Martha would share this information with her sister, this was a clever move on Freud's behalf.

Acting as his friend; Dr. Muller

Freud added: I will go out and see him with Dr. Muller ... acting simply as his friend. As Freud explained: I made it quite clear to him [Ignaz] that I am acting simply as his friend.  
So, now, when it was time to consult a real doctor, Freud was no longer Schönberg's doctor, but a “friend”, no doubt, to avoid being accused of malpractice. Bizarrely, but not for a psychopath, Freud informed Martha that he was, much more shaken by your dark rings [under her eyes] than by the poor man's deplorable condition to which he, certainly, contributed. (1)

Dr. Heitler

As the editor of Correspondence 1892-1938: Freud Sigmund; Bernays Minna, 2015, informed the world, acting as a friend, Freud, went with Schönberg to see a doctor but it wasn't Dr Muller. Instead, Dr Heilter (1) was chosen. This is what the editor revealed about the visit:
On the 8th [of August 1885], Dr. Heitler examined him in Freud's presence, found tubercular chancres even in his throat, and confirmed the poor prognosis.
The poor diagnosis wasn't unexpected. After all, Freud's cure didn't do much good or Ignaz, rather the opposite. No wonder that, having just received a death sentence, that Schönberg. became very withdrawn, no longer confided in Freud.
Without a doubt, faced with the terrible outcome of Freud's treatment of him, by now, Ignaz knew that Freud had played him for the whole duration of his disease. Thus, Freud wrote to Martha that he was certain that the relationship was over.
Notably, the editor doesn't provide the source of this information. 

Astonishing looks

A week later, on August 14, 1885. Freud conveyed a piece of good news about Ignaz to Martha - no doubt he was lying – claiming that it was, Astonishing how much better he [Schönberg] looks; most of the symptoms have subsided; I don't think the end will come for some time.
Also, Freud blamed Ignaz' family - anyone but himself - for Ignaz's ill health claiming, that, the brother tried to clear himself of all responsibility for the neglect.(2)
It is not clear what neglect Freud was referring to, conveniently forgetting that it was he, not the brother, who was taking care of (or rather neglecting) Ignaz during his illness.

Aversion and egotism

Unsurprisingly, having his axe to grind, Freud continued to slander his “rival” claiming that, Schönberg, confessed to him that, he felt an actual aversion to Minna and hoped that she, would soon …forget him and that she will behave as an unattached girl!
Aversion and dislike of Minna? Egotism? Only hatred and disgust were missing! Freud was a nasty piece of work.
While it is true that it is impossible to know what Ignaz said, there’s little doubt that Freud's idea was both to denigrate Ignaz, and to prepare Minna for breaking off her engagement to Ignaz.
Further, Freud claimed that his former “friend” asked him: Don't you think that the psychic burden of such a relationship is too great for me? My egotism is beginning to assert itself. All I really want is to keep going for a few years.
Freud, as he claimed, was very considerate, almost crying crocodile tears when writing:
The poor man, I told him that of course the dislike he had taken to everything was the result of his exhaustion.

In his letter of February 7, 1886 to Minna, referring to, the news of Schönberg's death, Freud callously told her: Your sad romance has come to an end. And in an apparent attempt to extinguish all traces of Ignaz’s existence, Freud was advising Minna: burn your letters [from Schonberg].

Wonderful things

Tantalisingly, Freud urged the young woman in mourning to, think ...  what wonderful and extraordinary things may still happen to our little circle. (1)
When he was talking about their circle, Freud meant, of course, a triangle consisting of, in the first place himself, the owner of the future harem, his beloved Minna, and, last and least, the ugly duckling, his fiancé, Martha.
And when it comes to those wonderful ... things that may  still happen, whether Minna realised it or not, there’s no doubt that Freud was talking (and dreaming) about pleasures of the flesh. As it turned out, when that day came, Minna didn't mind, on the contrary.

Moral weakness

One of the most recent Freud biographers, Crews, having provided his own translation of the above-mentioned letter, stated that Freud's letter, tactlessly referred to his dead friend's, moral weakness and rigidity. (2) Notably, those two offending expressions, reflecting Freud's true nature, had been removed from the published official English version of the letter.

Curious things

Even though he was writing to Ignaz' fiancé, Freud, which is consistent with his malicious character, was happily spitting on the dead man's grave whom he once called his "friend". Moreover, Freud urged the "widow" - in the Crews' version of the statement - to look forward to unspecified “curious things, worthy of being experienced. (3)
What those curious things were, a day came when Freud was happy to show her. Remarkably, this sensual letter was written only a week (!) after Ignaz’ burial on January 31, 1886. With his rival gone, Freud couldn’t wait.
            Besides Freud’s tendentious letters, there’s no information about the circumstances of Ignaz’ illness and death. An article in Neue Freie Presse, of February 16, 1886, talks about Schönberg’s achievements and death, but doesn’t mention the illness he suffered and died from.
Notably, it took only a couple of years, since Freud's diagnosis - despite, or maybe because of Freud's cures - for Schönberg to die. This was a remarkably speedy demise.  Did Ignaz really die of TB, or maybe his death, as in Fleischl's case, was somehow "hastened" by his doctor?
Had Freud neglected Ignaz, maybe even “poisoned” his victim – like he poisoned Fleischl - with cocaine injections - or maybe this wasn’t your standard TB, but a much more aggressive disease that killed him.

In the same letter, of February 7, 1886, Freud warned her that,
We are dealing no longer with him but with his family, and from what we know of them it is best to break off all relations.
Freud acted like a dictator. Ignaz was dead, and every ties to his family had to be severed.
Not unexpectedly, in anticipation of accusations of malpractice, Freud didn’t want Minna to have any contact with her former fiancé’s family. From Freud's point of view, there was a good reason for this kind of abrupt severing of all ties to the family of his deceased fiancé. 

Freud the cause of his illness

This is how Freud informed Minna what to expect,
You may as well be prepared within a few weeks or months to hear the whole family saying (and believing) that … my medical treatment … was the cause of his illness.
Without a doubt, this preemptive statement reveals the sordid truth about Schönberg’s death. Eager to get rid of the rival - as apparently Schönberg’s family was aware - Freud, no doubt, did all in his power to speed up Schönberg’s death rather than to keep him alive which is consistent with Freud’s constant denigration of Ignaz in his letter to the sisters, Minna in the first place.

Ask for photographs

Moreover, Freud told Minna that he would, without any delay (and without asking her permission). write to the brother [of Ignaz]… and ask him to send … the little case with your photographs, if it still exists. The man is fairly susceptible to the duties of a gentleman, and I hope to attack him from this side.
This is a remarkable statement! It is Freud rather than Minna that decides what should happen to her photos she gave to her fiancé. And, Freud’s planned “attack” on the bereaved brother, to have him accede to his wishes - just as all his earlier scheming to estrange Ignaz from his fiancé - was simply abhorrent.

Quack Freud

Strangely, except in one case, when Schönberg assisted by Freud visited a doctor, there's no mention of Ignaz being treated by any other doctor than Freud, neither in a sanatorium, nor in a hospital, only receiving a mistreatment by the quack doctor Freud.
Was Freud a quack doctor, indeed? As Freud informed his boyfriend, Fliess, on August 29, 1888, only two years after Schönberg’s death, he had, not learned enough to be a medical practitioner. *
And this was the man who treated the poor Ignaz. No wonder, Ignaz was doomed from the start.  Quackery and sexual desire are a deadly combination.
* Freud, Sigmund, The Complete Letters of Sigmund Freud to Wilhelm Fliess, 1887-1904, (1985, p. 23).

Unfortunately for her, Minna was callously groomed from the start by, not only older but, infinitely more (sexually) experienced Freud, as a result, never finding a partner in life other than Freud.
Minna was an attractive young woman …, and she undoubtedly had every opportunity to … marry sooner or later. But she was not really free; for years Freud had confined her within the triangle she so fittingly describes as “Martha and you and l." (1)
Without doubt, as the result of his efficacious manoeuvring, Freud cunningly managed to lock up Marta in the tower of his perverted love.
Shortly before his marriage [on September 14, 1886, [and only eight months after Ignaz' death] ... Freud courted his future wife’s sister - sometimes openly, other times covertly, but always with carefully chosen words. Minna was likely receptive to his warm words and advances. This is how, Freud, ensnared Minna. (2)
No wonder that being caught in Freud’s spider web of “love” and lies, she couldn’t find a partner.

Love a tergo

In 1896, thus, ten years after Schönberg’s death, when Minna -aged 31, still a spinster – although maybe no longer a virgin - moved in to live permanently with the Freud family, Thus, Freud's dream of having his own harem became reality. At least for Freud - since he managed "to get the girl",  or rather both girls - the Schönberg affair had a happy ending.
Referring to his "dream", allegedly involving his wife, Freud revealed that,
on the dream-day a girl [Minna] had come to live in the dreamer’s household who had attracted him and had given him the impression that she would raise no great objections to an approach of that kind.
What kind of approach was he intimating at? As Freud explained, the dream was,
a representation of an attempt at coitus a tergo (between the two stately buttocks of the female body). (3)
Now, Freud would be able to enjoy the pleasure of the other side of the moon courtesy of his sister-in-law,

Blood and fecal matter

Freud claimed that the dream was only about vaginal, rather than anal, sex, but, taking into account his coprophilia, his claim may not be entirely true. (Already the fact that he provides this kind of assurance makes his claim suspicious.)
A case in point: as Paula Fichtl, Freud family’s housekeeper recalled,
Martha Freud had explained to her when she started the job that Fichtl would sometimes find flecks of blood and fecal matter on the bedsheets, on the Professor’s side. (4)
Whose blood and faecal matter Paula was finding in Freud’s bed is anyone’s guess.
Notably, Freud, the notorious liar, to prevent the identification of the "girl", claimed that the woman (i.e. Minna), whose buttocks irresistibly attracted him, came, of all places, from Prague.

No sexual attraction

Funnily, Freud’s authorised biographer, Ernest Jones, the liar from Wales, (5) as Freud referred to him writing to Max Eitington, on November 27, 1927, claimed bizarrely that even though,
Minna and Freud got on excellently together. There was no sexual attraction on either side. (6)
But Freud, considered himself lucky to have “such a beautiful sister-in-law”, telling to Minna, “You know how much I love you". (7)

The Letters of Sigmund Freud

The available information about Schönberg originates in Freud's correspondence with his fiancé Martha, as well as with her four years younger sister, Minna. The letters edited by his son, Ernst, were published under the title, the Letters of Sigmund Freud, in 1960, 21 years after Freud's death.
The exact date when Ignaz Schönberg's relationship with Freud began isn't known, but his name is mentioned for the first time in Freud's letter to Martha of June 19, 1882. Freud's and Schönberg's relationship was to continue until Ignaz's death, on January 29, 1886. (1) 
And the first of Freud's published letters addressed to Minna was dated February 21, 1883. (2)

The Complete Letters of Sigmund Freud to Wilhelm Fliess 1887–1904

The published, Freud's, allegedly, complete correspondence with his boyfriend, Wilhelm Fliess, began with Freud's letter to Wilhelm, on November 24, 1887, and ended with Freud's letter to Wilhelm of July 27, 1904. The published correspondence contains only Freud's letters and covers 17 years, Thus, Freud's letters exchange began a year after the death of Freud's "friend", Ignaz Schönberg in 1886. 

Minna in the letters to Fliess

Even though Freud corresponded directly with Minna, at least since 1883, her name was for the first time mentioned in Freud's letter to Wilhelm in 1894, thus, first seven (!) years after the start of his correspondence with Wilhelm. Without a doubt, since it is virtually impossible that in his letters to Fliess Freud wouldn't even once have mentioned Minna's existence, letters, or passages in the letters, are missing,
So where are those letters, if the correspondence, as it was alleged, was complete?
A case in point. After the first mention of her on May 21, 1894, in which Freud referred to her as, my sister-in-law Minna, otherwise my closest confidante. (3) Minna was frequently mentioned in the letters.
Thus, three years before becoming part of  Freud's harem in 1897, Minna already replaced Wilhelm in Freud's affections
On November 29, 1895, Fliess was informed that, Minna came a few days ago for a stay of several months' duration. (4) Apparently, this was a "test run" of the harem. Remarkably, such a significant event as the fact that Minna moved in permanently with the Freud's, thus joining Freud's harem, in 1897, was never mentioned in the letters (or those letters are missing).