Settlement Query

Hello, I’m faced with a bit of a conundrum. We have a house in a very prominent locality in Chennai where my mother currently resides by herself. My family and I stay nearby. The issue is that about 13 years ago, my mother was afflicted with cancer and was not expected to survive. It has recently come to light that my brother tried to make use of this inopportune moment to get my mother’s signature in a ‘settlement’ which deems her share of the property when sold, go to him. My mother cannot recall this ever happening as she was put under severe medication and thus all she remembers was signing the document. I also have a recording of her making this statement as she is very ill, even now. My brother lives in the UAE and comes down once every year and is not aware of recent developments. I also understand that a ‘settlement’ once made, cannot be revoked under any circumstances. My mother’s only wish is that we both receive an equal share in the property. For the life of me, I am appalled and still haven’t come to terms with my brother’s actions regarding the matter. What is my recommended course of action? Thanks in advance.