Wolf In Me

Harry had a sister that no-one knew about because James and Lily never told anyone about her. She was three years older than Harry, but Voldemort never noticed her the night he came. She was turned at a young age by Greyback, but her lycanthropy was different than anyone else's--she can turn completely into a wolf, but she can keep her mind as if she was on wolfsbane. What happens when she gets forced onto the Hogwarts grounds because something chased her, wanting her dead, and Harry finds her?
Warning: Mature just in case. it's gonna be very violent, so don't read if you don't want to. Possible Trigger Warnings, but I wouldn't be able to tell what they are. Be careful reading, y'all.