and I know you're good
Sherlock, unfortunately, is used to getting the tiniest of details incorrect. There's always something, and he knows and accepts this phenomena, but that doesn't mean he likes it any more when he's wrong.
"Harry's short for Harriet," John tells him, a small sort of smile on his face that Sherlock doesn't have time to analyze.
"The sister," he snaps, mostly to himself. "There's always something." John nods along with him and the issue passes, and Sherlock files away the information deep in his mind palace.
---
Months pass, John and Sherlock fight crime together, try to take down Moriarty together, live together, eat together, (they don't sleep together, for what it's worth, but Sherlock wants to, wants to like nothing else in his life) and when Moriarty pops up again, going on and on about his Final Problem, Sherlock knows someone's going to get hurt.
So, he plans it all with Molly and throws himself off a building and spends the next three years of his life tracking down Moriarty's web and pretending that the photo of John at his funeral clipped from the newspaper isn't burning a hole in his pocket with every step he takes.
---
John punches him in the face when he returns.
"You bastard," he spits, eyes blazing as Sherlock gets off the ground, dusting his legs off. "You can't just- what makes you think you can- what in God's name makes you-"
"John?" The voice comes from the door, and Sherlock's head snaps up. It belongs to a young Asian woman, (Chinese, Sherlock thinks, but he can't be sure), who's pretty enough, but how on Earth does she know John?
"Yeah, Harry," John answers her, rubbing his hand. "Just fine. Just taking care of some old business, don't worry."
Sherlock feels sick suddenly, as if the ground were torn out from under him. That's Harry? Harriet Watson? She was definitely looked younger than John, but no, no she was definitely the same age as him. He could see her in the way John held himself, not the military rigidity, but something softer, more open. She must have been adopted, then, that made the most sense.
"John," Sherlock croaks, but John turns sharply on him, sticking a finger out.
"You don't get to speak," he says, full of venom and bite, and this must be one of those bad days he'd always told Sherlock about.
Sherlock nods in agreement, and they stare at each other for a long moment until John turns again, walking toward the house. Sherlock follows, and Harriet gives him a strange look as he limps by her, but does not say a word.
---