Brother-in-Law harassing/beating my sister very frequently

It's very hard moments for me to write this concern and writing with tears in eyes. My sister is frequently beaten up very badly by her husband for no reason, and he use to use very abusive words at daily life even. No normal person can accept the words that he uses. This is happening since years. He is so much convinced with his sister and brothers words, who is also such kind of persons. As of today, my sister is in the position with finger prints turned black on her face, with low BP due to hard hitting and on bed. He is not even having the courtesy that she is a arthritis patient. Before writing this concern, we have approached the relatives also to solve the problem, but it's not really helping to come out of the situation, since his brothers are bit in high positions (shamelessly, they are Govt teachers) and they indeed convince the relatives whom we are talking to - they do have businesses together. In fact, we don't have other relatives who is out of his brothers influence. And, we are not in the situation to go for divorce as well, we can't even think of it. It may be due to family status in the society or something etc., I'm suggesting, to file the complaint against them, including his brothers and sister, but my mom and dad are not willing to due to it may create another problems from their side like they will not compromise later for any solutions (of course, it's not working out), and he may take revenge on my sister by harassing again. I want to suggest to my sister itself to give counter attack (in the form of self defence at least during his attack again, using a small weapon in worst case), when he is trying to abuse her again. Now, I'm really confused that, whether it may lead to any legal problems. Though we are not having any other problem to be happy, this guy is screwing up our happiness and my mom and dad are literally crying whole day. They lost their health as well due to this guy. Please suggest us how to really get rid out of this problem and what could be the best solution out of police station.