Maybe try calling a domestic violence hotline? You can generally remain anonymous and receive some very helpful advice. It seems like you are stuck. Depending on your emotional state I would say, taking care of yourself is the most important. It also sounds like your sister is very isolated and has no one to confide in...
That's a tough one, if you feel strong enough to support your sister, I think she and the baby need help. If this is taking too much of a toll on you, sounds like your mom is a good resource for your sister.

Hope this helps....CSC