Divorce on the grounds of harassment and demand of dowry

sir as my sister is married for past 10 years from the day 1 my brother inlaw and his family demand for dowry as they need a site and as i wanted a legel advice that if i want to prove them they r guilty can i go to my sister neighbour hood and ask them to them to help me by writing the same as the above as those people have seen it as my brother in law tourcher and locks my sister in home and so on and he too have given us in writing that he are his family members wont demand any dowry ,please give me a advice