My sister sleeps at a house where an abusive ex-military male lives with his wife. This man has drugs and is dangerous. What can I do?

I assume your sister is an adult.  If that is the case, you cannot do anything except try to talk to her and encourage her to leave.  If she wants out of the situation, it will be up to her to take the action.  She can move somewhere else or if she is on the lease, she can complain to the landlord about the danger of the other man.  The bottom line though is that all action has to come from your sister and as long as she is an adult, there is nothing you can do if she chooses to put herself in an unsafe condition (unless you wanted to try to have her declared incompetent or something like that).  If she is a minor, it would be up to her parents or guardian to take action.  Even if you are a brother, if she is a minor and you are not her parent or a court appointed legal guardian, you do not have any right to tell her where she can go or what she can do.