I totally get the moral objections and the religious issues. BUT. To refuse to attend a sibling's wedding is so hurtful and seems like the worse evil ("sin"). Sister is basically cutting ties here and of course this will divide the entire family. I think she will be able to reconcile her Catholic heart if she attends the wedding for the sake of family harmony. If she truly can live with herself after boycotting brother's wedding... Well she is not worth much as a sister or as a Christian.
It seems like some people take their religious beliefs to a point that Christ himself would not approve of - but this is just a guess. I follow the compassionate and forgiving Christ myself