Rules for auctioning a property in case property values more than loan

My father built a house using all his savings and of post retirement. After his death, my mother owned the house. My younger brother mortgaged the property for a bank loan by taking my mother as guarantor without our permission. Promised to pay back but later refused. Now bank is auctioning the house. House's valuation is more than bank's loan, yet bank undervaluing the house. Bank says amount left post auction to be given to my brother, but my mother never made a will. My share is being denied. What can be done? We are caste hindus