You're the daughter of a death eater, you're a slytherin. But one day your mother gets send to prison so you're all alone, thats when the ministery decides to put you in a sort of foster care, they want to put you in a family with a normal surrounding. That's how you end up at the Weasley familie's front door. After you knock, Molly opens the door, wearing an apron. "Hello dear, you must be..?"