Harry Potter and Nyra

This takes place in the second book of Harry Potter. A young girl named Nyra is Harry's sister. The force of Voldemort's spell backfiring from Harry when they were babies knocked Nyra out the window into a forest, where she's lived in ever since. (Weird, right?) She gets an acceptance letter from Hogwarts.(I would've used the first book, but it's not my favorite)