Husband cheating on wife, forcing her get out of house

Dear Sir/Mam, This particular situation is pertaining to my sister's married life. Her husband has been cheating on her for some time with another married lady within the family. This is not a suspicion anymore as we have got some evidence of their conversation which pretty much describe their intimacy. The irony is the husband's family also stand along side him on this as he is the highest earner in the family and commands some degree of domination. As a family they have started abusing my sister mentally and at times physically which has resulted quite a bit a disturbance in her life. She has two children and she is scared to come out of the relationship fearing the future financial uncertainties in raising the kids. This has given the advantage to her husband and he has started doing everything openly without any hesitation. He claims to have few known people in police Dept and always threatens awful consequences if these things come out in open. He had another questionable relationship with one of his married cousin previously too which eventually came out when my sister got to read some of their private conversations. I have heard so much about human rights, child and women welfare. What possible solutions can this problem have ?