Mentally harassment by in laws

Hello Sir/Madam, My sister is married since December 2013 and she is pregnant now a days. Her in laws mentally harass her everyday since her marriage. Even they threaten her that she has to obey every order and has to stay with them though she always provide them whatever they like or whatever then want. They are treating her like a maid. They have made her to leave her home and now she is staying with us since last 15 days. They clearly told us that they wanted their son but not daughter in law. She just wanted to stay with her husband, separate from her in laws to have mental peace. Her husband also wanted the same but his parents are threatening him too, so her husband is not taking any decision. They are also demanding for divorce in this situation. Now her husband is not ready to take responsibility of the child. Please suggest what should she do in such situation. What if they file an divorce case in this situation. They are not even responding to our calls. Thank you in advance for help!!