1. Your sister should collect evidence and file a DV case against his bhai,
2. Is her father supporting her? If yes, then he should interfere and tell her bhai to stop harassing her or leave the house,
3. You should also collect/record enough evidence of such threat of the bhabi to lodge false complaint of sexual harassment against uour husband,
4. You husband should lodge a police complaing about her such threat of false complaint,
5. If any cush police complaint is lodged against your husband, he should immediately file application for Anticipatory Bail.