What to do in case of domestic violence and torture from in-laws

Hello Sir, My sister got married 2.5 years back, after few months of marriage her husband started beating her with no reason. Such things happened many times in last 2 years but last month he has beaten her severely and even stepped his foot on my sisters neck. 10 days back when we continuously asked the reason she sister told that her husband is psychologically unstable even before the marriage he is undergoing treatment but my sisters husband and her mother has hidden that from every one including my sister.After few months of marriage when they visited doctor (my sister was told they are visiting for other health problem not psychological issue) doctor spoke to her then she realised why her husband is visiting doctor regularly. My sister was forced to keep this as secret by her husband and mother in law. My sisters husband & family has cheated us by not reveling his condition before marriage and after marriage he started harassing her. Could you please suggest us next step to take