Harassment by In laws and Husband

Namasthe Sir/Madam, My sister got married during May 2016. Before marriage the Husband and in-laws spoke nicely with our family and my sister. They even took some amount of dowry from us (note - while taking dowry the husband and in-laws have signed on the paper in the writing the amount and things taken.). But a week after marriage they have started her harassing mentally by asking more dowry (in form of cash, silver idols, gold, land). They are behaving with her like she is a servant and also tell her that she is like a object purchased from market. Her Sister in law frequently visits their home and initiates quarrels among them. So out of all these problems my sister came to our house. After waiting for one month and in this period her husband and in-laws neither telephoned nor came to our house to take her back. But when we called they spoke rudely and demanded for extra dowry. One day we planned to sort things by having a meeting with them face to face along with our relatives and friends. That day they accepted their mistakes and told they will look after my sister very well and never ask dowry again. Things went fine for a week but after that again they have started harassing her. Her husband is doubting her and spying on her whenever she goes for job interview. After this we have told our sister to stay in our house. But till now the husband nor the in-laws have contacted us. So Sir/madam Please suggest what further actions we can take? or do we have to wait for some more months.