I'm gonna go with drug dealer brother who's forcing/threatening her to sell the drugs on campus for him. Maybe it could even be one these things where she does it because she feels like she owes him for something that happened when they were growing, like protecting her from and abusive Dad, or something like that. But even though he's now gone down a bad road, she still feels indebted to him and so has to help him even though she knows what she's doing is wrong, and he might also be the only family she has.
Would give her character a lot more depth if it were something like this, and might give people a fairly compelling reason to want to help her.