Deceased nominee

You shall obtain the death certificates of your mother and brother from the concerned.  Then apply for the legal heir certificate from the Tahsildar concerned.  After obtaining the two above, one of the legal heirs can approach bank authorities for the purpose.  And the FD amount goes to all the legal heirs in equal shares.

Obtain succession certificate from competent court through a local prudent lawyer.

It all depends on the quantum of FD.   According to my knowledge, all legal heirs of the mother are eligible for such a claim.   There is no need for nominees details.   However, the legal heirs of the nominee may appoint an attorney on the behalf of your deceased brother. You have to contact the bank and file claim form with original death certificate and depending on religion you may have to file prescribe documents for getting such a claim.  Your banker is proper in explaining the laid down procedure.  All three brothers and two sisters are entitled to the amount.

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The deceased brother is survived by his wife and two adult children. Will they also become legal heirs of the said property? The two sisters are married and settled elsewhere. Can we include them as legal heirs as well?

If you are 'Hindus", all siblings are co-sharers including sisters.  The share of deceased brother should be further divided for his legal heirs.  This is not our issue, the issue is satisfying the bank and then the co-sharers as their signatures are required for proper discharge.

Dr vashist has rightly advised you

Obtain succession certificate from competent court.

Please approach the Bank Branch concerned along with the Death Certificates of your Mother and Brother (Nominee). They shall guide you further course of action depending upon the quantum of the Deposit. You may do the needful as per the guidance given by the Bank Officials.

Yes, seek guidance from the Bank. all the legal heirs are entitled to share the fund equally. The deceased elder brother's legal heirs are entitled for one share(jointly).