What to do if my husband is having an affair with his brother's wife?

hindu was married in 2009 lead a happy married life after one year 2011 i given birth to a female baby... my husband is my relative my dad sister son..in the year we dont speak his mother and his family due to some mi sundering between son and mother from 2011. 2014 month of june he got left side plagiarized in goa when he was in navy.i was only looking after him alone with the kid ...and my parents supported me finance for his treatment...his medical state was very serious doctors kept him death list.As a wife i want to inform his family informed his mother.they came and saw for 2 or 3 hour .after that problem started between we both sir as he was retiring from service 2016 .we both have been staying separately only i and my kid is staying with my parent past one and half year he never turned back sir. he is staying and having affair with his brother wife and both are in same house in absence of his brother .his mother is supporting ...now i dont know what to do pls help me