Spoilers! The Ending

I can't believe they survived at the end and everyone else gets killed. At first I was feeling sorry for the victims and was looking forward to the sister's death at the end (at least the mean one). But by the time the end came around and the mean sister was killing all of the frat boys and the police were shooting it out with the biker gang I lost it and was enjoying the carnage. Good movie with a wild ending!