The best option for you is to be as genuine with your sister as possible. Explain to her what you see and what leads you to believe that she is being manipulated. Be kind and calm when you tell her. Express that the reason you are telling her this is because you love her and want the best for her. Tell her that you are on her side and that this is just how you see it. Be open to listening to what she has to say back. Be open to the possibility that you could be mistaken (not saying you are, just saying to be open minded).
If she will not hear what you have to say then there isn't much else you can do. Don't try and force her to listen, if she won't listen you can't do anything about it. Trying to force her to listen will only make her not want to listen even more. This being said, I would say it is unlikely for her not to hear you out if you tell her the right way (the kind way).
If you feel comfortable (if not that's fine too, you don't have to do this) you could talk to her boyfriend about the situation. You could make it clear that you are aware of what he is doing and that you wont stand for it (stay away from physical violence at all costs because it will only add difficulty to the situation).
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