Like others have said, you can try talking to her if you feel comfortable enough to do that. It sounds like she is in need of help, and if you were able to get her to talk to a mental health professional that would be good. It isn't just the cutting and yelling that are signs that she desparately needs help - the way she so quickly took to sleeping with your brother is another danger signal. I bet that she does not think much of herself at all. You are not obligated, of course, to put up with her behavior or to care for her. She needs more than she can get just from a friend. If she continues to do these things you can call emergency services - the police and/or even health emergency services, and ask them to help you deal with her and to take her out of your home and see that she gets the proper assistance.