cuz the guy she was screwing (her ex-girlfriend's brother. Her girlfriend dumped her for guy and ended up breaking up with him to go for ANOTHER girl) chose his girlfriend over her.
Of course, she one-ups him because the girlfriend of the brother ends up dumping him anyway and then she rejects the brother and leaves. And then the sister gets the parents to stop sending money to the brother so... yea
spoiler alert!She ends up with the brother in the end anyway. Not because of love, but because they feel comfortable with each other