Need advice for my sister on next steps.

My sister got married in January(arranged marriage) and my father being a cancer patient never informed the groom family about the two roka cancelled in the past as they never asked . The groom got to know about it after marriage because he was always checking my sister’s phone and messages on social media on all platforms . He also checked all messages from her friends before marriage for past 2-3 years. The groom is now unable to move on and thinks my sister had other relationships and now he is not treating my sister right. We don’t want divorce but we also don’t know what to do. She is depressed and we have tried everything to prove to him that there were no past relationships other than 2 broke rokas and he has also not found anything after marriage but he is struck on her past and thinks that whatever she is saying or has said is a lie . My sister is unable to talk to us as the phone is always monitored and recorded by him. I want the advice on this situation.