It is what it is

Important note: these are taken from the script. As such, they may or may not appear in the final scenes.

A STUDY IN PINK

It wasn't me who upset her, Mycroft.

appears to be a very obvious line at first. However, technically, this implies nothing about who did upset her. It is an indicator, however.

Onwards.

A SCANDAL IN BELGRAVIA:

Mycroft: I'll be mother. Sherlock: And there's a whole childhood in a nutshell.

This clearly implies that Mycroft functionally raised Sherlock, or at least was a significant carer for him.

Mrs Hudson: Family's all we have in the end, Mycroft Holmes. Mycroft: Oh, shut up, Mrs Hudson!

It's easy to dismiss this as him being rude, but, considering -- his family clearly doesn't have an issue with abandoning him, as he well knows. What Mrs Hudson says can't be true to Mycroft, because if so, he's got nothing.

THE EMPTY HEARSE

Sherlock: Can't handle a broken heart. How very telling. Mycroft: Don't be smart. Sherlock: Oh, that takes me back. "Don't be smart, Sherlock. I'm the smart one!" Mycroft: I am the smart one. Sherlock: I used to think I was an idiot. Mycroft: We both thought you were an idiot - we had nothing else to go on. Until we met other children.

Let that line sink in for a moment. They didn't meet other children until they were old enough to remember such things. It is worth bringing out a quote from the ACD canon, spoken by Holmes in The Adventure of the Copper Beeches:

It is my belief, Watson, founded upon my experience, that the lowest and vilest alleys in London do not present a more dreadful record of sin than does the smiling and beautiful countryside...But the reason is very obvious. The pressure of public opinion can do in the town what the law cannot accomplish. There is no lane so vile that the scream of a tortured child, or the thud of a drunkard's blow, does not beget sympathy and indignation among the neighbours, and then the whole machinery of justice is ever so close that a word of complaint can set it going, and there is but a step between the crime and the dock. But look at these lonely houses, each in its own fields, filled for the most part with poor ignorant folk who know little of the law. Think of the deeds of hellish cruelty, the hidden wickedness which may go on, year in, year out, in such places, and none the wiser. 

At the very least, this is indicative of children with nowhere to turn to if abuse should happen. Combined with the hints of neglect seen elsewhere, one wonders as to how much responsibility Mycroft was forced to take on.

Sherlock: Oh, yes. That was a mistake. Mycroft: Ghastly. What were they thinking of? Sherlock: Probably something about making friends? Mycroft: Oh, yes. Friends.

Consider this with knowledge of what happened to Sherlock's first friend, and it makes you wonder -- did Mycroft realize what Eurus would do? He is the smart one, after all. Did he try to avoid having friends so Eurus wouldn't kill them?

Mycroft: I'm not lonely, Sherlock! Sherlock: How would you know?

It seems likely to me that Mycroft deliberately keeps people far from him, so they don't become targets.

Sherlock: Mycroft promised to take them [his parents] to a matinee of 'Les Mis'. Tried to talk me into doing it.

In fact, the scene at Les Mis is played for a joke.

Mycroft: But you don't understand the pain of it! The horror!

And it's true that what we see doesn't seem so terrible. But there is another element. It's possible that they are being abusive to Mycroft, but not Sherlock (well, except for the whole 'ridiculous levels of isolation' bit alluded to above). That these are merely further occasions for them to hurt him.

THE SIGN OF THREE

Mrs. Hudson: Your mother has a lot to answer for. Sherlock: I agree. I have a list. Mycroft has a file.

Again, it's supposed to be a joke. But the idea. That he has a file. And in context, it's about Sherlock's manners and lack of knowledge of the world. But we don't know that's what it's about. Possibly it's about that. Possibly it's about their abuse. Possibly it's about their neglect.

HIS LAST VOW

Mycroft: I shall have to phone our parents, of course. In Oklahoma. It won't be the first time your substance abuse has wreaked havoc with their line dancing.

The way he says it, it sounds like it's coming from him. But that doesn't sound like Mycroft. He wouldn't care for line dancing, I imagine; he would view it as a distraction, and caring for Sherlock as paramount. Anyway, he knows guilting Sherlock won't work.

So most likely, he's repeating what someone else said to him. Presumably, his parents, asking him why he interrupted their line dancing for something as unimportant as their son taking drugs.

And then Christmas.

Mycroft: Dear God, it's only two o'clock. It's been Christmas day for at least a week now, how can it only be two o'clock. I'm in agony.

Again, you're supposed to laugh. And that seems to be a habit -- Mycroft will say things like that, and you go ha-ha. But what if you take them seriously? What if that's why he dislikes Christmas -- because he has to spend time with his family, and be reminded of how they hate him, no matter what he does?

Sherlock's mother: Mikey, is this your laptop? Mycroft: On which depends the security of the free world. And you've got crumbs on it. Sherlock's mother: Well, you shouldn't leave it lying about if it's important. Mycroft: Why are we doing this? We never do this. Sherlock's mother: Because Sherlock's home from hospital and we're all very happy. Mycroft: Am I happy too? I haven't checked.

Am I happy too? I haven't checked.

I'm not over that line yet.

And then, immediately after:

Sherlock's mother: Behave, Mike. Mycroft: Mycroft is the name you gave me, if you possibly struggle all the way to the end.

One presumes he's voiced this before -- it seems unlikely to be the first time he brings it up. But either way, she doesn't apologize for it.

Sherlock's Father: Gave it all up [her career] for her children

What, abusing them? I find this difficult to believe. Presumably he was frequently absent, to not notice such things.

Sherlock's mother: Are you two smoking? Mycroft: No! Sherlock: It was Mycroft.

Mycroft denies, but Sherlock blames Mycroft -- because, of course, he knows Mycroft will take the blame, cover for him.

By the way, a very fascinating script divergence in the scene in which Sir Edwin goes: "If this is some expression of ... familial sentiment ...", Mycroft replies:

Mycroft: Don't be absurd. You know what we did to our sister.

Not strictly relevant, but I thought I'd mention it.

THE ABOMINABLE BRIDE

Regarding Victorian!Mycroft's heart:

Sherlock: There’s only an empty cavity where that organ should reside. Mycroft: It’s a family trait.

To be fair, this is all in Sherlock's mind. But even so...family traits are generally inherited.

Presumably, from their parents.

Mycroft: We have an agreement, my brother and I, ever since ... that day. Mycroft: Wherever I find him, in whatever back alley, or doss house, there will always be a list.

Wherever I find him. There's no indication his parents search for him. We don't even know if they realise he's missing. It's all Mycroft's job, searching.

Mycroft: Sherlock, listen to me. Sherlock: No it'll only encourage you. Mycroft: I'm not angry with you. Sherlock: Well that's a relief, I was really worried. No, hang on, I really wasn't. Mycroft: I was there for you before. I'll be there for you again. I'll always be there for you.

And he is, and no one recognises it, because they assume the rock will always be there.

Mycroft: This was my fault. Sherlock: It was nothing to do with you.

His automatic taking of the blame for Sherlock's drug usage. Sherlock's immediate deflection, a more touching moment: This isn't your fault, there are some things you can't control. Probably as close as he ever gets to I'm sorry.

THE SIX THATCHERS:

A clinical, cold kitchen. Things still bubbled-wrapped. It’s never used. MYCROFT comes in. He rubs his eyes wearily, goes to the fridge. Nothing in.

First of all, note the indication that his eating disorder extends in rather the opposite direction. Second of all, how absolutely miserable his house is. This seems the place of a workaholic, or someone rather depressed -- it's certainly not a sign of stunning mental health.

And either his parents don't notice, or they don't care.

THE LYING DETECTIVE:

Mrs Hudson: Anyone who stays here a minute longer is admitting to me, personally, that they don't have a single spark of human decency. [Mycroft stays.]

Oh, Mycroft... it's worth noting he leaves when she tells him to, calling him a reptile. But this quiet declaration, that he believes himself to not have a single spark of human decency -- someone's put that thought in his head, or, if it put itself in, failed to help him.

THE FINAL PROBLEM, AKA: HOO FUCKING BOYYYYYY

First of all, the starting scene reflects poorly on John's character, as he's clearly isolating Sherlock and getting Sherlock to distrust another major figure in his life, i.e., Mycroft. But we'll ignore that; this isn't a discussion of John's abusive tendencies.

Mycroft: Mother and father were terrified, they thought it [Eurus cutting herself] was a suicide attempt - but when I asked Eurus what she was doing[...]

Now, there are two likely options here, I think.

Their parents didn't bother to actually ask why their child was attempting to commit suicide, or what she was doing, or anything. And if not that, then why anything else?

Mycroft was Eurus's 'interpreter', i.e., given the responsibility of translating for her and explaining her actions. This is probably better, but even so, disturbing considering the information she'd be relaying.

Finally, note that he never specifies they. Household staff? Their parents? Someone else? We don't know. Given the fact that he promptly says 'Mother and father' shortly after, if it was their parents, it would make sense to say 'Mother and father found her with a knife once' and then say 'They were terrified', but he doesn't, which is an implication that it wasn't their parents who found Eurus.

Mycroft: This [her death] is the story I told our parents - to spare them further pain and to account for the absence of an identifiable body.

Why was he the one telling him? If it was Rudi/Rudy -- the spelling is inconsistent across episodes, but it seems to be Rudi here -- who made the decision, why was Mycroft saying it? Was Mycroft involved? Judging from their appearances, he'd probably be fifteen or so at the oldest. He should not have been responsible for that.

When Sherlock adds that he told them this to "prevent their further interference", he agrees:

Mycroft: That too, of course.

He shouldn't have been preventing it. Why was he responsible for preventing it? Why was he responsible for any of that?

Mycroft: Oh God. I might have expected this. Pathetic. Eurus is right. You always were the slow one. Always the idiot. And that’s why I’ve always despised you. You shame us all. You shame the family name. Now for once in your life do the right thing.

But...he doesn't despise Sherlock. He's made that quite clear. And yes, he's lying. But there's something else.

You shame the family name.

Sherlock doesn't seem the type to put much stock in that.

You know who does?

Mycroft.

Again, someone's said something like this to Mycroft before.

(Maybe just his own head. But in that case, no one noticed.)

Mycroft: Well, I suppose there is a heart somewhere inside me. I don’t imagine it’s much of a target, but why don’t we try for that?

This echoes the empty cavity scene in TAB.

Mycroft: Good bye, brother mine. For the greater good.

For the greater good. That's why he thinks he should die. Because it's the greater good.

Lestrade re Mycroft: Bit shaken up, that’s all. She didn’t hurt him - just locked him in her old cell.

If she didn't talk to him, he'd still be alone, with no knowledge of where his brother is and with a psychopathic sister on the loose, for hours. And if she did...well, just because he's harder to manipulate, when his self-esteem is practically negative, it's got to be easy.

Bit shaken up. Clearly not -- Mycroft must be a mess, internally at least.

Hell, he must be a mess for the whole thing! Of course he is! He's not an Iceman! And Sherlock can throw a tantrum and John can have Sherlock and Mycroft -- Mycroft's not able to break down. Mycroft's not able to show his emotions.

Now for The Big Scene.

The family scene.

Script:

Mrs Holmes in full flow. Her anguished husband at her side, Mycroft blasted by his mother’s wrath. Sherlock sits to one side, silent.

And then

Mycroft: What Uncle Rudi began, I thought it best to continue -

When? It seems like Mycroft was involved in this very early, when he was still a kid.

Mrs Holmes: I’m not asking how you did it, idiot boy. I’m asking, how could you??

Idiot boy. When we hear Mycroft in Sherlock's mind palace, Mycroft belittles him using very similar terms. If anything, he's gentler. This lends itself to a few possibilities:

Mycroft was communicating in the way he knew how, as a child -- and he learned it from Mrs Holmes.

Mycroft was in charge of relaying things from Mrs Holmes to Sherlock, and tried to report them accurately, which leads to Sherlock attributing them, at some level, to Mycroft.

Sherlock's clear viewing of Mycroft as a parental figure extends to ascribing to Mycroft things his parents said to Sherlock.

Sherlock overheard Mrs Holmes saying things like this to Mycroft as a child, observed their effect on Mycroft, and decided to try it as a means of getting him to do things.

None of these are good options.

Mrs Holmes: Then you should have done better. Mycroft, about to speak. Realises that’s crushingly right.

HE WAS A FUCKING CHILD.

And if, at best, this is meant to be 'Mycroft thinks that's crushingly right' that still shows an alarming degree of parentification. He was, again, in his early teenage years at the oldest. That is not an age to be managing the incarceration of your psychopathic sister who everyone in your family but you and your uncle thinks is dead.

Mrs Holmes: Well then, he’s very limited.

I note, by the way, that Mycroft seems to have been the only one to. You know. Respond. Him and Rudi. The Holmes 'parents' seem to have done next to nothing. We hear them arguing at one point about how to make Eurus tell them where Victor is, but that's it.

Mrs Holmes looks at him coldly - her disappointing son. And looks to Mrs Holmes: Sherlock? Well? You were always the grown-up. On Sherlock[,] realising, to his own surprise, that that’s true. Mycroft, slightly lowering his acknowledging the same.

A claim that is not only ludicrous, but suggests that they don't even recognize their parentification.

In The Sign of Three, the moment immediately preceding Sherlock's comment on Mycroft's 'file' is the realization that Sherlock thinks tea has just been appearing out of nowhere, rather than Mrs Hudson making it.

It appears that Mr and Mrs Holmes took much the same approach to their children's parenting.

CONCLUSION

Essentially, there are almost no positive interactions between Mycroft and his purported parents. What we see, instead, is a pattern wherein his family, particularly his mother, persistently ignores his desires on things as minor as what he'd like to be called, places massive responsibility on his shoulders and blames him for not managing it perfectly, passes off most if not all of the work of dealing with Sherlock's self-destructive tendencies to Mycroft, and isolates him from seeing any other child for at least the first seven years of his life, probably nine at minimum seeing as Sherlock would also need to remember to know that!

This is a fairly clear-cut case of psychological abuse, probably neglect, and -- honestly, I don't even know what to call the isolation Mycroft grew up with. I genuinely do not know. It is absolutely insane.

And they get off with nothing! No one objects to this! No one says, "Hey, what the fuck are you doing?" No one.