My sister wants to get married?

Unfortunately you cannot do anything. She will have to learn about life the hard way. Trust me she will eventually realize he is abusing her when she gets tired of being isolated. That is the first sign of an abuser. And the reason he is so unsociable is because he knows what he is. He will make her miserable and she will get sick of it but not if you attack him. If you do that she will just defend him more and be more determined to prove you all wrong. She just is not very smart. She is using him too, to escape. She will someday learn to appreciate what she has, it just will not happen anytime soon. Just be there for her when she needs someone and decides it is time to escape from him.