Mental and social harassment

Dear sir This is on behalf of my sister. Actually she was in relationship with a boy for 3-4 month last year(they were in physical relation too.)but after spending some time with that boy in livein she realized that boy is not fit he is mentality syco. He start abusing my sister and about my family and infact he starts physical harassment. Then she inform me about her mistake and request for help. As being from girl side I suggested her to leave him immediately and end all the contact from him as I m afraid of our family image in society. But after 1 month of this incident, some he get some of my relative and family persons details on fb. He start sending them mgs about their relationship. After that we spoke with him and with his family to solve out issue and make them understand the things. But as I mentioned he syco, he was remain silent for few days. After that again he starts the same things even this time he starts sending abusive Mgs to my parents. Now he is forcing us for my sister's marriage with him so that he can destroy her life.kindly advice me how can I get rid off this. If we going for police, he is saying that he will kill my family as his father is builder and having strong contacts. Pls help me.