Nomination only entitles the nominee to receive such funds from the concerned authority, but that does not mean that the legal heirs would not have a right to receive them. You should ascertain some facts first—did he leave behind a will? If he left behind a will and bequeathed such funds to his sister, you would have to challenge the will when probate or letters of administration is filed by the sister. You should also get a succession certificate in the meantime to claim your rights. As long as you were not formally divorced (i.e., a decree obtained through courts) and there is no will, you/your children (if any) are class 1 legal heirs and have rights over and above his sister. However, I think you would not get the EPF, gratuity, commutation, etc., without a succession certificate.
Regards, Meera Kaura Patel
Partner, LawCircle
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