For years Mycroft simply assumed he was adopted. It seemed the logical conclusion: he'd been unwanted by some strange, cuckoo-peculiar set of genetic antecedents, and adopted instead by two utterly ordinary, adorable people who were, poor things, doomed to put up with him: Mycroft. As he suspected he was better off as the cuckoo in the nest, he was hardly going to complain, though sometimes it was unnerving to find himself watching over his parents rather than the other way around. But it was hardly as though he didn't owe them the care in return for their generous decency in putting up with an obvious Ugly Duckling rather than a proper, ordinary child.
When Sherlock was first born, Mycroft considered it a long-overdue return on his loving parents' patience and good will to him, and he swept the skinny, colicky infant into his family willingly enough. Granted the child was loud—oh, goodness, was he loud. And stubborn. And even aggressive, once he started to crawl and explore. He could grab Mycroft's ear faster than Mycroft could dodge away, and would then yank and yank, trying to remove it for better inspection. As for hair? Mycroft years later occasionally claimed that he lost enough hair to constitute foreshadowing of Mycroft's own eventual receding hairline…if not qualify as just the first incidents of direct cause and effect.
Then, though… Ah. Then. The realization that there was not one cuckoo in the nest, but two: or, more precisely, that "adoption" was not an answer that could be relied on. No. The entire adoptive hypothesis eventually had to be dumped entirely, as it slowly became obvious that Sherlock was, like Mycroft, a very odd duck indeed. Barring a theory of complex conspiracies in which the elder Holmeses were repeatedly duped through baby-switches in hospital, there was nothing to do but conclude that somewhere, somehow, these two gentle people had come to carry a set of genes unexpressed until their match-up. A set of genes doomed to inflict the most preposterous of offspring on them.
Mycroft was older, at this point; rising on eleven, with Sherlock turning an obstreperous, difficult four. It was becoming obvious that Mummy and Daddy had no idea what genetic karma had handed them. Mycroft, after all, had been a pleaser, convinced that even if his adoption hypothesis was false, his darling parents were deserving of every effort possible on his part. He tended to obey rules, respect limits, and stay within boundaries, even as his mind hurtled at light speed to the outer edges of reality. He could travel the universe while remaining very still and minding his manners as he tagged along with Mummy at the local Sainsbury's. As a result Mummy and Daddy were in no way prepared for the same bounding, leaping mind in an equally bounding, leaping body, with grabbing hands, flying feet, and a set of lungs that would do an opera singer proud.
Mycroft, for the first time, rather resented Mummy and Daddy. Or resented their limitations. No eleven-year-old likes discovering that Mum and Da need his help to deal with the sprawling genetic disaster they've set loose on the world, yet it was so clearly obvious they were out of their depth entirely. And they doted so… all that genius was rather adorable in Sherlock, where it was merely quietly present in Mycroft, and Mummy and Daddy took such pleasure in Sherlock's wildest antics and proclamations. Things Mycroft would not have done or said—would never have done or said—were treasured up and passed to all and sundry when Sherlock did them and said them. As a result Sherlock tended to wallow in his exceptional self a bit much…and caused endless rounds of domestic and public difficulty that distressed Mummy and Daddy as much as they charmed them. The two parents floundered, unsure how to even predict Sherlock's wilder excesses, and with no idea how to contain them.
They were, instead, rapidly turning their new son into an ongoing series of phone narratives to friends and relations that always started with the laughing, horrified, frustrated words, "You'll never guess what Sherlock did this time!"
Mycroft, by then, was old enough and educated enough to imagine them as the first poor primate couple to be handed a recognizable human infant thanks to the genetic crap shoot. "Can you believe it, Mabel? That crazy kid actually collected fire and brought it in the cave! And you don't want to know what he's started doing with flints, for God's sake. I cut myself on this thing he'd been fiddling with the other night. And he leaves the chips all over. I just don't know what to do with him!"
Only, the boy thought with quiet, desperate frustration, the first little genetic advance in the family had been him, and he'd known better. Why didn't Sherlock? To this he had no answer. However he did know that, unlike Mummy and Daddy, he could sometimes predict, and almost always understand what Sherlock would do…and that he could usually determine a way to contain it, at least a bit.
In spite of the entire adoption theory crashing and burning, Mummy and Daddy were still the people who'd taken Mycroft in and kept him against all good sense. Nothing would ever quite take away that feeling on his part: that he'd been blessed to land, a cuckoo child, a peculiar genetic fluke, in the nest of two such loving parents. He wanted to care for them, as best he could. And Sherlock, for better or worse, was Mycroft's own—more so even than Mummy and Daddy were his own. Bone of his bone, flesh of his flesh, mind of his mind, a wild, out-of-control, demonic reflection of everything he was, both for good and for ill.
Someone had to take over the parenting of Sherlock when Mummy and Daddy reached their ordained limits.
What followed was, perhaps, unsurprising: the tangle of love and annoyance, resentment and competition, trust and betrayal. No eleven-year-old, cuckoo's child or not, really wants to deal with a squalling, high-tempered, uncontrollable little brother. Not really. Not entirely. And it's frightening and disturbing to see your worst potential played out before your eyes, over and over. And it's almost impossible not to resent, just a little, the parents who can't quite stretch far enough to cover the entire obligations generated by their own reproductive madness.
But Mycroft was Mycroft—the oldest son. The good one. The quiet one. The one who understood rules. The one who fought to please.
And Mummy and Daddy were his beloved parents, so gentle and accepting and ordinary and kind.
And Sherlock was…Sherlock. Brother and bete noir. Mirror and nightmare vision.
Someone had to watch over them all, and make the world safe for lunatic little families of proto-human primates and their neo-human offspring. And if not Mycroft, then who?