I had a similar experience - not the same, but comparable - with my best friend (we are like sisters, I have known her since she was 11, we were at boarding school together). It's one perspective which might help you a bit in your awful situation with your sister.
Her "partner" wasn't physically violent -yet- but he was a deranged, controlling bully who was displaying all the classic behaviour of isolating her from her friends and family, emotionally blackmailing her, constantly chipping away at her conidence etc. DH and I tried for about 2 years to encourange her gently to see what was happening and persuade her that he wasn't right for her - I didn't want to come on too strong because I was the only friend he hadn't yet banned her from seeing and I wanted her to have somewhere to come when things got really bad, which they were definitely going to. Anyway there came a point at which he had started to try and phase me out as well - finding fault with everything about us including our parenting and our lifestyle, picking fights and generally ruining the atmostphere whenever they were here. She stopped calling me because he made her feel uncomfortable about it. Shortly after that I told her (by email) that I was ging to be briefly in the town where she lives - for a few days - (we live 200 miles apart) and would she meet me for a few hours, just for a drink or lunch ot whatever. She told me that she couldn't, because it would "make him very unhappy". It transpired that he didn't allow her to go out without him apart from to work (she worked to support them, he didn't work), no work parties, no lunches with friends, nothing unless he was there.
I felt really torn at this point, but decided there was nothing to lose as he was going to stop her seeing me anyway. I wrote her a long and very blunt email explaining what I thought of their relationship and how worried I was for her future. The idea of her having children with this person, being tied up with him financially and emotionally for ever, made my stomach turn. I'm afraid I used a little blackmail myself, in that I told her that she had always been one of the most giving and loyal people I had ever known, a really good friend - and as long as she was with this man she couldn't be that sort of friend to anyone apart from him. I half expected her to say "well, I'm with him, so if that's how you feel you can sod off". Instead she left him, the following week, and she hasn't looked back. She is now working for her own upkeep rather than someone else's, lives in her own nice flat and has a boyfriend her own age who treats her with respect. She was desperate to leave him anyway, had found herself wishing him or herself dead, but had been convincing herself that it would get better. She says my being blunt with her helped give her the impetus to move.
Sorry this is a waffle, but your OP rang bells with me - I know you don't want to lose your sister, and it is a terrible position for you to be in. I thought my experience with my friend might help a bit.