Absence of marriage certificate

You may file an affidavit as to the effect of marriage and submit it before the concerned authority. Otherwise since your uncle was a government servant he must have mentioned these details in his service record so your aunt may obtain the certified record from his office.

Take a copy from service record of deceased and submit to the Registrar of Marriages with an affidavit.

Ration card, Bank account, insurance policy, Aadhar, Voter card etc., are some bonafide ID documents.  Through that and with the help of those colleagues who attended the marriage, you can prevail upon employer for such confirmation on affidavit.   The Pensioner's Association or his colleagues will certainly help in sorting out the problem.   Take the guidance of his colleagues.  Most of the employees then had traditional marriages, and such a marriage certificate is unknown.  There was never any need for even declarations and just informing name of wife as nominee, recording relationship was sufficient.

Registering marriage is not mandatory as of now in our Country. Therefore, no Department can demand the Marriage Certificate to pass on terminal benefits of a deceased employee, including sanction of family pension.

There shall be many attendant documents where your Uncle might have given nomination:

1. For PF and Gratuity to the Employer
2. List of Dependents to Employer
3. Bank Accounts where Nomination is given duly giving the relationship
4. Post Office Accounts where Nomination is given  or
5. Insurance Policy where the Nomination is given in her favor

Any one of the above documents plus a notarized declaration to the effect that your Aunt is in fact the wife of your deceased Uncle as per the procedure of the organization where your Uncle had worked.