He's seven when he makes his first deduction, and he immediately wishes he hasn't. Mummy says she's sorry, but he's not sure he'll ever believe a word of what she says from now on.
Father doesn't say anything, he merely looks sad; and old, far older than he actually is. In the end he simply forgives her, and they never mention the unfortunate slip again.
Until little Mycroft makes another deduction, one that is even more unpleasant to his young mind.
"She's your sister, Mikey," Father tries to explain. "We have to take care of her."
"No, she's not. I don't want her. You shouldn't want her."
"It's the right thing to do."
"I don't care. This is just stupid. Get rid of her."
"Mycroft," Mummy chides him, but he simply refuses to listen.
"I hate you," he tells her, then flees to the safety of his room. He won't live under the same roof with a half-sibling that is the living proof of his mother's inexcusable behaviour, he just won't.
His parents reluctantly give way, start making arrangements for the baby girl to be put up for adoption when the time comes. Mycroft doesn't feel any remorse whatsoever, and 'the other one' is quickly forgotten.
Three years later, when Sherlock is born, he willingly takes up on his duties as a big brother, determined to shield the little one from any pain Mummy may unwittingly bestow upon him.
xxx
He never tells Sherlock about their half-sibling, nor does as much as lifting a finger to find out where she might be at this moment in time. His baby brother is difficult enough to deal with, considering his crazy stunts and his history of recreational drug use.
Over the space of a year and a half he watches Sherlock fall in love with his new flatmate, almost lose a battle of wits against the most dangerous woman he'll ever meet, then jump off a rooftop in order to save his friends. After that comes the two-year mission to dismantle Moriarty's network, and Mycroft's subsequent trip to Serbia in order to extract his little brother from what certainly qualifies as a touch-and-go situation.
He has enough on his plate without having to witness Sherlock make a fool of himself upon his reunion with John Watson, not to mention when he agrees to be his friend's best man. There's something nagging at the back of his mind, something that Mycroft can't quite pinpoint.
It's too late when he realizes how dangerous Mary Watson actually is, what a ruthless woman hides behind the façade of the doctor's wife. He knows it's her as soon as he's reported about Sherlock being shot, though he chooses to deal with her on a later date.
Of course the silly brat has other ideas, almost kills himself in a pointless attempt to show he can fix everything on his own. That's when the former assassin contacts Mycroft, tells him she has a matter of the utmost importance to discuss with him.
He reluctantly agrees, and later on he has to admit he wasn't quite prepared for the kind of revelation she had in store.
xxx
"Well done, brother dear," Sherlock acknowledges as he blows out a puff of smoke. "Bringing back Moriarty was a nice touch indeed."
He only raises a questioning eyebrow. "I've no idea what you're talking about."
"Please. We both know you can't afford to lose me. My loss would break your heart, remember?"
Mycroft takes a drag on his cigarette. "I recall saying something along those lines, yes. Brotherly compassion is a powerful motivator, I've been told. However, I'm afraid I can't take the credit for saving you this time around."
"If you say so. Who's my knight in shining armour then?"
"Hardly a knight, though brotherly compassion can be found in unexpected places sometimes."
His brother scoffs at such statement. "Well, I'm sorry to disappoint you, but I only have one sibling."
"Do you really?"
Sherlock's cigarette is now smouldering between his fingers. "What do you mean?"
"I'll let you guess, brother mine."
Silence stretches between the two of them, until the truth dawns on his little brother at last. "Is she…?"
"I guess Dr Watson does have a type after all," Mycroft sighs as he drops the cigarette and carefully treads it out.