Sirius Black has a twin sister who always want to impress her parents. So when they get accepted into Hogwarts their parents are happy for them. So when Sirius is sorted into Gryffindor he expects his sister to be happy for him but all he see's is disappointment. Sirius's sister is sorted into Slytherin and he's a happy for her. Since Sirius didn't write to his parents, his sister did instead and they send their son a howler. Sirius's sister meets lots of new people but she and Severus Snape fall for each other. Her parents don't care that he's a half-blood since he's a Voldemort follower. Marauders Era.
25 parts