Engagement and then cancelling marriage

My sister got engaged with a guy 4 years back, and post engagement the boy went to abroad for studies. We asked the boys family to marry my sister as soon as possible but the boys family promised us that once the boys is back from abroad after studies, he will marry my sister. It took him 4 years to come back to india and then a date was fixed for wedding. Now only two months were remaining for marriage (from now) and all of a sudden boys family refused to answer our calls and messages. They are now behaving awkwardly and the boys father told that his son is no more intrested in the wedding. Now We have a huge doubt that the boy would have married when with some other girl in abroad and just was wasting my sister's time all this 4 years untill he realised that wedding is going to be real and then he panicked and cancelled the wedding. These criminals have wasted four years of my sister. She is undergoing emotional breakdown and is getting depressed day by day. Please help with what kind of legal case we could file (on behalf of my sister) against the boy and his father for such a inhuman act.