You won’t be able to physically end her relationship with him, you being nice to him wont change the way he treats her, being nice to him is only making sure you are not adding fuel to the fire. You might want to consider seeking a professional counselor or at least discuss that option with her if she would be willing to go with you to one that you both can thus discuss the situation in a productive manner with a professional. If that is too expensive, it will never hurt to seek the aid of local priest to guide you, the help is free and all you have to do is schedule a time to speak to one. Your sister is the one who has to end the relationship, and probably to be prepared to seek legal council an get a restraining order.
All you can do is keep yourself open to communicating with your sister, being nice to someone does not mean you have to like the person, especially someone who is hurting your family. You can only give your sister options, and do your best to encourage her to at least hear what her options are and that you will be there to support her the entire way.
Catholic Charities has plenty of resources and I am sure your local community has plenty of directories for battered women.
But the first thing to do is pray, pray for peace for the both of them, and pray that Christ will free her from the pain she is in. An not just one prayer, but endlessly until you see the change that your sister needs, so that she may live in peace and find real love, good love but better the Love of Christ. If you find it in your heart to pray for him, it wouldn’t hurt, as his life is probably more messed up than hers’ and he is just bringing her down with him.
