Don't screw up your life and do what most of us would want to do if we had a sister who's husband was hitting her. Unless you catch him in the act, you are not doing anything to stop an immediate threat and you will be the one going to jail - he may go as well, but you would for sure. Call the local P.D. where she lives, advise them of the situation and see what they can do. If he is a convicted felon, his word will not mean as much as that of someone that is not.

Good luck, and prayers going out for your sister that she, or the unborn baby, don't get seriously injured by this a$$hat!!!!