Fixed deposit nominee update

Hello , My Mother in law passed away recently and My father in law is still with one of their sons (only two sons). After her death the two went updated the FD nominee equally (split half) in the Indian bank along with my father in law and kept their original recipts with them. My father in law is now 82 years old and he became critically ill recently beacause of his health conditions and is suffering from some memory issues. In December 2020 my husband and I went to the bank and came to know from the bank that my brother in law changed and updated his name as nominee to all of our share saying that the originals were missing. At that time the bank manager told us that my father in law even can not sign so they take the thumb impression and they did not let us know while changing to his name. We were so surprised hearing this and totally got confused on this. Please advice us how to proceed on the matter here. Thanks