Dear Diana,

I am 18, my sister is 17. I think my sister's boyfriend is cheating on her. He already had a girlfriend and my sister knew this. However, she has no problems with that girl. I think he never makes an effort to understand my sister.

She is busy studying these days and doesn't receive his calls, so he starts shouting at her. My sister cannot even go out anywhere with my parents without "his permission". She is also not allowed to go to any of her girl friends' houses. Most of the time, they argue over trivial things and past occurrences. He is a year older than her. How can I save my sister from him or make my sister understand what kind of a person he is?

Name withheld