Marital Distress

My sister has a major problem. She is married to our cousin. The story is very long, but I will tell you a brief summary. He doesn't respect her, speaks very badly, avoids having sexual relations but still wants a child, masturbates excessively, chats online with other girls (we don't know if he meets them or not), and has been sending money to other girls. Once, during a conversation with another girl, he said: 'My wife (my sister's name) is divorced if I don't send you money every month.' After all these issues, my sister stayed at her parents' house (maika) for six months. He did not give my sister any expenses (maintenance) at all. We had kept the dowry (Haq Mehr) amount high, fearing he wouldn't divorce her—this fact was confirmed to us by his parents. He would tell my sister to waive the dowry, and then he would give her a divorce. After six months, because he is a cousin and the family would break up, a reconciliation was made. It was agreed that he would give her 600 Riyals per month for expenses and stop all his other bad behaviors. In return, his demand was that my sister should visit her marital home once or twice a month. My sister went back, but he reduced the pocket money to 200 Riyals, which he gives with great difficulty and sometimes not fully. He still avoids physical intimacy, continues to masturbate, and is still chatting with other girls, and this time, everything is even worse. My sister is 22 years old. She is unable to separate due to the family situation. He has clearly told her several times that he does not like her. What should be done in this matter? There are no chances of him improving. Please guide us on what should be done about this problem.