Aleykum selam, brother
It saddenes me to read about this. And what's even more sad is that this happens everywhere and often. May Allah subahan we ta'ala give the suffering ones strength, guide them and help them in hard times. Amiin.
My mother was in an similiar situation with my father, his sisters, brothers and his mother. My Dad paid very little attention to these things, 'cause he worked all day in the factories that he owned and came late home and spent little time with his family (as he was just married he rather was with my mother). Things happened behind the back of my Dad and my mother didn't dare to tell him 'cause 1) he'd firstly believe his dear mother 2) he had such a temper, he'd blow things up and she didn't want to destroy his relations with the family.
What my mother did, was tell her own family and didn't take things in,she fought back, simply. The families met, my father had no clue and a huge war broke out. But elhamdulillah my father understood the seriousness of the situation and in the blink of an eye they moved to an own apartment. causing much bitterness for his family, which I still can taste today.
Also, I've got an cousin in the exact same situation as your sister. Things are bad, really bad. She hasn't seen her family in over 2 years, and they live just a few hours away. Not call, she barely manages to chat through MSN with her sisters. Her father got really mad, was ready to kill her husband, who is also an abuser. After that he wanted to order someone to kill him. But we all managed to calm him down, but now.. Now he tries to forget things and never mentions her name. He's angry and her mother is devestated. They are clueless what to do. May Allah subahana we ta'ala give them all and those in similiar situations strength to over-come the hardships and help to make things get better, forgive their sins and purify their hearts! Amiin.
My first advice would be to call the police, for some help, if things really get out of order. For my cousin it didn't help, the police take money and shrug your call for help off.
Firstly your sister must talk to her husband and get some support by him and her family, you all must talk to HIM, not those in-laws. He's the husband, he's gotta be able to handle his family. And if not, they must move away. Such things won't only hurt the wife and their relationship, but also her comming pregnancies inshaAllah if they want to have children.
My mother was under constant stress and emotional abuse, two children she lost due to it. When I was born, it changed my father to the fact he finally opened his eyes. Now, 16 years later in Finland, he's an very practicing Muslim elhamdulillah and I am more than just proud of him. He listened to my mother and the good relations now, are thanks to them communicating and Allah subhana we ta'ala.
Maybe you all should (surprise) visit the family, the uncles, the brothers, mother, sisters and have a good talk. Or call the husband and your sister over for dinner, and ask her to come visit you, rather than she asking them for permission. Always talk to the husband, make him realize he has power, not his family. The communication between husband and wife is the A and O for an successful marriage.
May Allah swt help her marriage and give some guidence to her inlaws and and help and give is guidence anybody else in similar situations.
Amiin, amiin, amiin!!