Harry Potter and Big Sister

It was 7 years since they first met, a little boy whose life was miserable in hands of his caretakers, and a girl on the search for her way home. Friendship fostered in short time, little boy always saw her as a bratty-yet-kind-hearted big sister. Then, the girl has finally found her way back home. Now, before 5th year of magic school he attended started, he meet her again.