This is the story of how Mycroft found out that his brother was still alive. He wasn't told his brother was still alive but, as it is difficult for a dead man to demand your help, he surmised this to be the case on receipt of Sherlock's note. Just ten short words, which told him everything he needed to know and ensured his compliance - you had to hand it to Sherlock, he could be economical when it counted.
A 221B for Starry's Birthday. Happy Birthday, Honey. =;-D
Lie - it is me - all their lives depend on it.
That was all. That was all the note said. Scrawled in barely legible script. And Mycroft lied - yes, that's him, he'd said nodding slowly; looking somber, looking penitent. Not devastated, that would have been overdoing it a tad. Caring is not an advantage - oh, how that phrase would haunt him.
Since then there had been nothing. Well, not nothing precisely, there were the unexplained deaths. Not how they had been killed - poisoned, strangled, stabbed, shot - but why Moriarty's lackeys had been killed and by whom.
Mycroft suspected that there wasn't another man alive who would have taken this occurrence as he had. Anyone else would be outraged, shocked, hurt - one of a number of emotions that he did not feel - concerning the manner of being told. He felt, as much as he felt anything, that this was as it should be. Not to be told, but expected to cooperate, comply with Sherlock's nameless plan. This is just how it should be, for the order of the universe to remain intact.
One day, one unexceptional day, it struck Mycroft. No one else would have reacted as he had. Sherlock had known that, trusted him implicitly to safeguard the vulnerable. And, in that moment, predictable was all it took to be Sherlock's brother.