The banks in the due course, as per laid down norms for the settlement of the claim in case of a nomination, performed their duties correctly. You ought to have anticipated such clandestine moves of your brothers when you are aware of the fact that your mother has apportioned jewelry.
First...Remember that a Nomination is a contract between depositor and bank, and not in between legal heirs as family settlement.
Hence if it is mentioned in a will but not given or closed as per nomination, you have to contact a local advocate and demand for partition of all the properties through notice and take appropriate action.
You can show the copy of wills in your possession and let your advocate decide the validity of that will.