Brother secretly mortgaged our shared flat in Mumbai - what can I do?

I just found out my brother mortgaged our jointly-owned flat in Mumbai without telling me. We inherited it from our parents, and he didn’t mention anything about needing funds or taking a loan. Now, the bank is sending notices, and I’m worried about losing the property. I trusted him, but I feel betrayed and don’t know my rights here. Can I challenge the mortgage or take any legal action against him? What steps should I take to secure my share of the flat?