He's fourteen when it happens – the first clumsy attempt of a teenage boy at exploring his sexuality gone horribly wrong, all because of a faulty condom.
Their families hush things up, though he can't quite understand why Liz's parents urge her to keep the thing. It's not like a fifteen-year-old can raise a child of her own, and the poor girl runs away from home as soon as she's gone through the ordeal of childbirth.
Mr and Mrs Somers move to Scotland with their grandchild, and he never hears from them again. Which is for the best, because his plans for the future never included an unwanted offspring; and the very last thing he wants is his little brother deducing his far from edifying secret someday.
The apple doesn't fall far from the tree anyway, as he discovers some twenty-five years later. Miss Somers is one of MI6's best agents, and something akin to pride stirs in his heart; he knows that fatherhood is never going to be his thing, but he can still keep her as close as permitted by his current position.
"What's with you and your new PA?" his impossible sibling demands point-blank. "I thought you preferred men."
He all but rolls his eyes. "I do, and I can assure you Miss Somers is the very last person I would have intercourse with."
Give him a puzzle, and watch him dance – that has always been his motto when it comes to Sherlock, and he cherishes the rare occurrence of having just rendered him speechless. And if it'll take a lifetime for his little brother to figure out his actual connection with Andrea, he can't say he really minds.
It's difficult enough to keep Mummy from inviting her unofficial granddaughter to the Christmas dinners in the first place.