Your sister is an adult and she has a responsibility to herself and her daughter to get the **** away from this man. He is dangerous and he's nuts! You had a responsibility to call 9-1-1 when you were there and he wielded the knife. She can get into a D.V. shelter, if nothing else. Right now, despite what has happened, she is making a decision to stay with this man. And I can guarantee you, if she stays she is going to become a statistic. You don't want to call CPS because they can complicate things? Do you think they can complicate things further than if he murders her? Come on, get real! If you want to really help her, tell her that either she needs to make the decision to get herself and her child to safety, or you will have to be the one to make a report about all this. Give her a deadline, then do it if she fails to take action. This man is a ticking time bomb and he's not going to get better. Your sister might be pissed off at you for awhile, but eventually she will see why you pressured her the way you did.