Xanthe Potter & The Ignis Lapidem(Year One)

AU, may travel closely to cannon for the first 3 books, hopefully I'll be able to write all 7 years. What if instead of only one Potter, the prophecy pertained to two? What if Harry Potter had a twin sister named Xanthe, and there is more to the prophecy and the circumstances surrounding Voldemorts first fall than it seems. Rated T because I'm paranoid.