Hmmm, Sherlolly + Hogwarts AU because of REASONS
The They’re both in Ravenclaw, even though the Sorting Hat did consider Hufflepuff for Molly. But, well, there was this rude second year wearing Ravenclaw robes who’d deduced exactly where she’d lost Toby (her stubborn, shy half-Kneazle) and the Hat did take preference into account
The Holmes family is one of those snooty Pureblood families, except Sherlock’s parents are delighted at all things Muggle. Lady Holmes is heavily into Arithmancy and enjoys cooperating with Muggle mathematicians. Lord Holmes humors his wife and is happy to let his oldest son take on the politics aspect of being a Holmes. Mycroft is an Unspeakable at the Ministry. No one knows what he does, but he claims to be a low level clerk.
Molly’s mom is a Squib, and no one expected her to have any kind of magical gifts. But she is bright and highly motivated and hoping that she’ll feel like less of a freak if she’s surrounded by people like her. Magic is exciting and it can be used to help people. Her Dad is already sick, and well maybe...
Molly’s best friend is a Slytherin girl named Mary Morstan. She catches a lot of flack for being friends with a snake, but well, she wasn’t considered for Hufflepuff for no reason. She likes Mary, her quick wit and her skill at Transfiguration. They’re Potions partners and they drive the Professor absolutely crazy because even as third years, they’re better than most of the NEWT students.
When Molly’s a fifth year, there is a ball. She goes with Mary, and ends up sitting on the sidelines as a sixth year Gryffindor attempts to flirt with Mary, not knowing that Mary’s set her sights on him ages ago. Sure, she confides in Molly, that John Watson has a lot to learn still, but he’s stuck with Sherlock Holmes for this long, so that speaks for his loyalty, and he’s a skilled Dueller.
Sherlock is mostly on the periphery of Molly’s life for her first years at Hogwarts, even though she tries to engage with him. He’s helpful in the rudest of ways, and Toby had taken a liking to him right from the start, and he genuinely seems to like Mary for his best friend. But then comes fifth year, and Mary and John start seeing each other, and Molly and Sherlock end up on a lot of not quite double dates. Mostly Molly (and to a lesser extent Mary and John) end up tagging along on mysteries that Sherlock is just determined to solve.
The first time that Molly offers a helpful bit of insight that helps solve the case, Sherlock dismisses it as a lucky break. The second time, he takes a bit more notice, but it’s the third time that he decides that he will allow John to go on private dates with Mary if Molly helps him instead. She too is going to be a Healer after Hogwarts, except not so much in battle magic (John might just become an Auror instead), but in determining curse and potion damage.
Sherlock’s always known that Molly’s had a bit of a crush on him. But he’s never known what to do with that. Because his studies and his cases come first, and he’s just figured out who dosed that Hufflepuff seventh year’s pumpkin juice with love potion. And if that’s what love is, no thanks? Yeah, no thanks.
Even though in Sherlock’s Amortentia lesson, he just smells parchment and chemicals and... sour cherries?
When Sherlock is about to graduate, and his rivalry with the Moriarty boy gets out of hand, it’s Molly he turns to. Molly doesn’t tell anyone, not even Mary or John. So when Sherlock seemingly just vanishes after a Potions accident...
Molly’s seventh year at Hogwarts isn’t nearly as exciting. Except at her graduation, there’s a familiar tall figure waiting for her, wearing a ridiculously fancy coat.