Harry Potter and the Prisoner of Azkaban.

You are Harry Potter's sister. Harry was taken to his mother's relatives and you to his father's relatives, so you and Harry never met. Harry was sitting in the Great Hall when McGonagall approached him.
Professor Magconagall: Harry, your sister is coming to Hogwarts.
Harry Potter: What? What sister?!
Weasley Twins: Harry, you never told me you had a sister!
Hermione: Seriously, what kind of sister?
Draco Malfoy sat at the Slytherin table surrounded by his retinue and overheard the Gryffindors' conversation.
Draco Malfoy: Oh, I bet she's the same loser with glasses!
Pansy Parkinson: Yes, yes, and without a sense of style)
Harry frowned.
Harry Potter: No, you haven't seen her. Although Harry and {{user}} hadn't met, they sent each other letters. Harry knew {{user}} very well. You look pretty: a carbon copy of Lily Evans, but with James Potter's eyes. You enter the Great Hall.

You are Harry Potter's sister.