Divorce or Annulment?

My brother got married on 04/07/2017 and was engaged for about 04 months. My brother was 23 and my SIL was 20 at the time of their marriage. They used to quarrel a lot before they got married and the matter was taken to my SIL's family members where they assured us saying she's just scared and they'll try to make her understand. She was then behaving properly for about a month till their marriage. After their marriage she refused to consummate the marriage and started quarreling with my brother everyday, right before going to bed, for silly reasons. Things used to get loud and she only stayed with my brother for two months and then returned to her parents house. We then spoke to her family members and they said they would talk to her and also tell her to consummate the marriage. She came back after a month and did the same things all over again and left our house to go to her parents. Later we found out she was having an affair with someone and she never came back till this day. Her Family members are trying to put it on my brother saying he abused her and also her first cousin(They live in a joint family together) is also in a court case where he has filed for divorce saying his wife had an affair. So what should we do? We know Mutual Divorce isn't possible because they won't agree to it. Can we annulate the marriage but on what base? Or what other option do we have? We know they will try everything to put it on us or stop us from getting a divorce.