Bank fd

My mothr who is 96 years old with an unsound mind. She has FDs worth 30 lakhs in her name in two banks in Kerala. My sister in law and myself are the nominees in all the but two/three deposits. My mother is mentally unstable at times and very often talks irrelevalently to 'You Tube' about her childhood and old deceased friends. She does not remember how much FDs she has got and who are the nominees. There are 3 heirs to my father's and mothers property and bank deposits. I have two brothers, the eldest brother since deceased. We three, my second brother, my first brother's widow and myelf are legal heirs to the property and bank deposits. My mother is with my younger brother. He is trying to unscrupulously encash the deposits for himself, by threatening or by other means my mother. Mother is ready to sign anything for him fearing repurcussions. What are the legal remodies available with us to stop . Or a bring a legal remedy to establish that the fds could be legally shared aftr her death. An reply will be appreciated.