Harassment of wife by husband&in laws,husband extramarital affair

My sister married since 09 yrs with 3 children. husband is a doctor. in laws always stayed with her. They harassed my father a lot before marriage regarding dowry negotiation and finally somehow they were married. my sister was pursuing MCA before marriage which was stopped by husband forcefully and she was confined to the house without permission to even to go out of home alone and the in laws were made to guard her at home. she was not allowed to talk to anyone in his absence. so landline was disconnected and mobile was given to inlaws and not to her. anybody (including us) wanting to talk to her, she had to attend in their presence. husband had an extramarital affair with a relative of ours, which we all (my parents and we 2brothers) greatly opposed and warned him to correct himself with arguments and harsh words. This hapenned at 4 yrs of marriage. Subsequently things were again normal. But he threatened our sister to cut off all relations with us with including tele communication failing which he would divorce her. We were also similarly threatened by their inlaws and their blood relatives (his brothers, sisters and their families). My sister was tortured every day and threatened to cut off relations and was confined to house. Many a times we tried to contact her but were not allowed by them. Thrice,she got pregnant (delivered twice with one abortion) and underwent choleststectomy. Also she developed hypothyroidism but we were not allowed to meet her or enquire about her health status, or to see the newborn children. Whenever we tried to enquire the relatives (we were not even allowed to talk to her inlaws to enquire about her health) they used to say that she is very happy and if we wanted her to stay happy with her family then we should forget about her and we had no choice but to abide. We had not met or heard directly about our sister and her children for next 5 years. Few months back her husband was caught in extramarital affair with another colleague (continuing for last 4 years), and that girl when started blackmailing him for monetary compensation, he spoke to us. He has dropped my sister and her children at our house. In no way does my sister want to leave her husband, but she has already suffered so much and he is unlikely to change his attitude in future also. How should we go about and ensure that her family is not destroyed