My sis husband doesn't support her. His sis and mother torture my sis mentally a lot. Her husband says whatever hid sis and mom says do so. She is in a big trouble please help me. What should be done?

Please clarify your question.

Some parts of the world get away with this. Please say where you are from and what culture or religion plays a part, and whether or not it was an arranged marriage.
If she left and went back to your parents, what would happen? Would the family be disgraced?
We can't help you without a LOT more information.
Even then, she is going to have to get YOU and your PARENTS to help her get out.

Joy is right. Unless she has the support of you and her family, nothing can be done.