Why Do You Hate Me?

Raina Snape is the twin sister of Severus Snape. She went to Beubaxtons for three years before having to go to Hogwarts with her brother. She has never met any boy other than her brother. What happens when she meets the Marauders? What happens when she becomes the fifth Marauder who knows every single spell, every single power? What happens when she's a werewolf? What happens when she has many powers she doesn't know of? What happens when her family forces her to become a Death Eater?
She's also an animagus and a metamorphagus.