What her husband is doing is very wrong in God's eyes -- not only by physically abusing his wife, but also by trying to control her by distorting God's words concerning marriage. The Bible says, "Husbands, love your wives and do not be harsh with them" (Colossians 3:19).
Don't misunderstand me; God takes the marriage vow very seriously, and I wouldn't want anyone to use what I say here as an excuse to break their vows. But God also values our lives very highly; human life is sacred in His sight because He made us in His image. When someone's life is in danger (such as from a violent, abusive spouse), then God would have them do whatever they can to remove themselves from that threat. (This doesn't necessarily mean divorce; it may only mean a period of separation.)
Most communities have an abuse hotline; encourage your sister to contact it so she can make wise decisions and find safe shelter. She may think that's a drastic step -- but it may awaken her husband to his own need for help. Bullies and abusers often act that way because down inside they feel weak or powerless, or they have other psychological issues they need to face.
Pray for your sister, and do all you can to support her as she faces hard decisions. Urge her, too, to connect with a church where she'll find people who will love her and help her. Most of all, remind her that Christ loves her, and He will never abandon her.