Mycroft had secrets, of course. Everyone did. And he was a Holmes after all, and they seemed more more prone to secrets. Not to mention his government job, which mainly consisted of making stories and keeping secrets, not that he was going to let Sherlock have the satisfaction of knowing that, obviously.
But there was one he knew would keep til the day he died, even after he was tortured and had all the others ripped out of him, god forbid it ever come to that, one secret, one lone secret would remain.
Because terrorists could steal military plans from him, rogue government agents could take the whereabouts of foreign diplomats, but no one, not even the best trained agents, could tear the secret from Mycroft he had kept for thirteen years.
He was the one who killed Father.
He could pretend it was an accident, self defence gone wrong, perhaps he fell down the stairs in a drunken stupor and snapped his neck, but he was far above that. Mycroft Holmes was not one to stoop to such lowly tactics. He was a Holmes after all.
He planned it perfectly, made it look like an accident. There was no need for him to lie or make up an alibi, because at the time of death, he was away at school.
Ingenious really. The old man would have been proud.
Or perhaps not.
Mycroft was not stupid, no matter what Sherlock would have anyone believe now. It was easier to keep up that front than to be forced to play his hand. But Mycroft had not made this decision lightly. He had examined all other options before finally deciding what had to be done.
Because he was fine when it was him, but when Father moved on to Sherlock, things went downhill rather quickly.
Sherlock never knew when to keep his mouth shut, or more likely, couldn't be bothered to, and that just made everything worse. Mycroft had become practised, learned to shut up and take the beatings, not to mouth off, knowing it would only make Father more angry. Sherlock went the opposite direction, working Father into a frenzy until he could barely speak for rage, unable to use words, resorting to actions alone.
Every time afterwards, while cleaning Sherlock up in the bathroom they shared, Mycroft would mutter to Sherlock about shutting up and not antagonizing him, because that only made it worse. During those talks, Sherlock never said anything.
Mycroft didn't know why he couldn't do that before and solve the whole problem.
And then there was Mummy. Father never hit her, never touched her, at least not as far as Mycroft could tell. No, Father saved his words for her, finally finding the ones he'd lost and wanted to use on Sherlock, taking them out later and blasting her with them, leaving her broken and bleeding behind ineffective shields.
Mycroft had tried. He'd involved police, tipping them off anonymously, but that was useless, Father was far too highly respected. And when the police did come round, Mother always denied everything, Sherlock didn't speak, and Mycroft lied through his teeth when they asked questions, pleading with his eyes for them to notice something was not right.
They never did.
So he had to take matters into his own hands.
It was easy enough, a short acting poison that would leave no traces and give him an alibi. Sherlock wouldn't be suspected, and Mummy would be too shocked to be the culprit. He made sure they had alibis anyways, just in case. It was perfect.
He got the phone call at school just as he suspected, managed to cry, and promised to hurry home.
The official cause of death was a heart attack.
As Mycroft held Mummy and Sherlock at the funeral, her crying and Sherlock expressionless, he knew he had done the right thing. But the gift he had given them, no one could ever know about.
It would just be his little secret.