Mental harassment

My sister has been married for last 18 years. The husband and his three sisters have been mentally torturing her ever since she got married. There's a servant family living with them. The husband favours the servant over my sister and his children for every small and big thing as the servant took care of his parents till they passed away. His sisters manipulate his mind against my sister for everything and he dances to their tunes and has terrible fights all the time with my sister, almost every time in front of their children. He abuses her, threatens her that go do whatever you want, I'll see what can you do, you can do nothing etc etc. He is a businessman and is loaded with numerous loans from here and there and keeps her beg him for money every time with a typed response of no money no money. He ill treats her in front of everyone including the servants family. He emotionally threatens her with his impressions created outside. She is mentally very disturbed. It's a daily thing for his sisters and the servant family. He keeps loading the talks that he will never leave the servants family as they have done so much for his family and that if my sister wants, she can do the "faisla" and call us and his own family, which I'm assuming for separation. But is it morally, legally right? Her children are 17 and 10. She doesn't earn anything, she's a housewife. She doesn't want a separation for her children's sake. Kindly tell what are her rights and ways to protect her, in case of any future legal and societal steps that her husband forces on her.