disclaimer: all harry potter stuff is J.K. Rowling's.
long story short, Harry Potter has a twin sister, Leilanie, who was put up for adoption a week after birth, and she grew up in Hawaii. The only thing she has from them is a locket with a 'P' incrusted in rubies on it. There is a picture of Lily a James in it.

other info: Leilanie looks just like Lily, but with icy blue eyes. She wears contacts.