Harry and Mira - Twins Through the Ages

What if Harry had a twin sister - Mirabella? How would this have altered the story line? This is an alternate universe about the twins, Hogwarts and exactly what would have happened with old Moldyshorts if he was battling not one, but two prophesied children. And also what would have happened if Snape recognized the twins who lived for who they were, and not over arrogant idiots.