Can legal action be taken against my sister's abusive ex-boyfriend?

My sister was in college and had a boyfriend who was very abusive, a drunkard, and used to beat her during their relationship. However, they broke up, and everything was going well. My sister got married and is now living a happy life. The problem is that, even after so many years, her ex-boyfriend still comes to our house late at night, fully drunk, and abuses our whole family. When I informed his family about this, they are blaming my sister. Can you help us with this?