Sister's in laws cheated and did'nt inform about her husband's disease

Sir my sister got married to a guy in nov 2015,before the marriage there happened an incident_few days after the engagement with my sister,her mother in law's family called and told us that their son was suddenly turned ill and the condition became serious.we have enquired them whether there was illness before and they said that it was not before and that was happened after engagement,then we went on for marriage After 2 months of marriage,he again turned ill to the extent where he was kept in an emergency and then from the words of the doctor,my sister came to know that he has been suffering from a serious heart disease from the childhood itself,he became so ill that he is unfit for having kids also and he may die anytime,so my sister wants to take divorce from him and wants to get back our money(dowry),but they are turning against us by saying that we have informed about the medical condition before but they didn't,they only told us that the illness happened recently,what to do?