Hanretta Black And The Serpent's Secret

Harry Potter has always known one thing the rest of the wizarding world doesn't: he isn't an only child.
Raised in secret alongside her twin brother by the last man anyone would suspect, Henrietta has spent her whole life learning how to disappear - how to count the days between Spinner's End and Privet Drive, how to keep Severus Snape's name out of her mouth in front of strangers, how to love her brother loudly at home and not at all in public.
When the Hogwarts Express splits them for the first time - Harry to Gryffindor, Henrietta to Slytherin under a name that isn't Potter - the secret gets harder to keep than either of them expected. Especially once Draco Malfoy finds her curled up alone in the common room after the Philosopher's Stone nearly gets her brother killed, and starts, without ever quite meaning to, taking care of her.
The Other Potter begins where the story you know left off - in the space between two children who were never supposed to need each other this much, and did anyway.