My sister husband need divorse but my sister don't want. She want to stay with her husband

Hi, My sister got married in 2009, she had one girl child and a boy. Before marriage they said, they don't want dowry but have to do marriage grandly. So we have done marriage very grandly. We have booked a function hall and showed them which they didn't accepted, we lost advance and gave chance to them to select a function. As per their wish, we have booked function hall for 3 days. A single rupee has not been invested by boy family. Everything done by us. We had kept gold in marriage, given money to brpther in law sisters. Fulfilled all the things they said. From the first week of the marriage they showed true colours. They always need money. They torture my sister always. First week after marriage they said, they need TV, Sofa, wardrobe, silver plate, silver glass, spoon which is about 1.2kg silver. We had given even. In the first week, boy mother heated a spoon and kept on my sister hand. When my sister asked a soap for bathing. They said, have to get soap by her own money. First delivery was a baby girl, so after delivery whole boy family came to our home and caught my father shirt color and asked why I got girl child. We have expected a boy child first. Why girl child delivered first, they fought with us. Every month they make one or some other issue even we had given money multiple times. Every month atleast once they hit my sister. For children they don't give good food. They will beat to children if they support her mother. When beating her mother son was crying, they said if he won't stop crying they will kill him. I used to order snacks for children as they won't buy and give. Even in the name of Yoga, brother in law has to take and come girls to home, lock the door in bedroom with girl. Always chat and talk on phone with girls till midnights. When my sister raise voice, boy parents say you also talk with other boys. As my sister having girls problem, they even didn't showed in hospital. My sister health was going worse, then also she managed some how. At last my sister was shown to homeopathy doctor, he said to take rest and drink coconut water more, even they didn't do that. They said coconut water is not good, you will get cold. She was in a position as she is not able to walk a single step. At that time, they want my sister to go from their home and stay in parents home. As children have schools, my sister thought that she want to manage the things. But they have booked the general sitting ticket for 280kms In train, when she said she will go by bus as after getting from train it is hard to come to our place so they said we won't book bus ticket. You want to go in train, whole family best my sister and my sister said don't beat I will go and she came by bus as by bus she can directly get down at our place. Even she don't know the train journey. Now they need divorse where my sister don't want as we are respectable family. Please help me what to do now