Husband and In laws harassment

My sister living in Hyderabad, A.P has been constantly harassed from the day of their marriage to this day (15 yrs). -The husband has taken all the money that was given to her during or after the wedding. -He is been having affairs with different ladies in the past 15 years. -He made my sister abort a baby after finding out that she was going to have a girl. -He and his mother have been threatening to divorce her or marry a second wife since my sister was unable to give them a son. They have 3 girls (13, 10, 5). -He has stopped her and still stops her from gaining financial independence by working even though she has a Masters in Chemistry and had many chances to be independent. -They treat her like a slave at home. Constantly verbally and sometimes physically abuse her. -She doesn't have any financial independence. -She wants to visit her mom overseas because she is sick. However, he is harassing her saying that he wants her to sign on blank bond papers before even she can appear for a visa. We(her siblings) are all living abroad and don't have much info how to help her. Since she is by herself, we don't want her to get into some more trouble after taking any steps. What is the best course of action in her case to stop the harassment and mental torture? How can we help from abroad? Please let me know. -Seema.