Tom-- Just don't get mad at the sister if she doesn't leave and/or takes him back. Most people take quite a few practice runs to leave a jerk like that. Always have your door open, always let her know she can rely on you for somewhere to stay and for financial help leaving. These kinds of situations have their own timetables and though it's easy to look at it from the outside and see that she should leave him it's not as emotionally easy for some people to do it.