JKR's mention of a "Mark Evans" in book 5 is not a coincidence. Evans is Lily and Petunia's maiden name. Either they had a previously unknown brother, and Mark is his son, or Harry's muggle grandfather, Mr. Evans, had a brother who had a grandson Mark Evans. Evans is definitely a muggle name, as far as we know, but what if Mark's father married a witch, making Mark halfblood? Also, Mark was ten in book five, meaning he'd be eleven and of Hogwarts age in book six. A coincidence? I think not. The whole prince business is less certain, as well as why Harry couldn't live with the Evans--the Dursleys were supposedly his only remaining relations. Still, it's the best theory I've got. Obviously, the book alone will tell all.