Which personal law applies in your case?
Are you all Hindu?
Succession opens on date of death ; by inheritance or by testamentary succession ( by valid WILL).
In case of Hindu male dying without disposing her estate/property in her life time by a valid/registered deed the 1st right is of ClassI legal heirs ie.e Mother (if alive as on date of death), Wife (if alive as on date of death), sons, daughters……
So if mother of your father was alive as explained above she also has equals share.
The authority under whose jurisdiction property falls has a set procedure for such matters if NO WILL has surfaced; Intestate Succession…….and the prescribed forms, procedure, process is available in O/o Authority and even on website. Death certificates, legal heir certificate/affidavit (per local procedure-precedence) are basic requirements. Then authority shall act upon and transfer the ownership in the name of legal heirs. Obtain copy of updated mutations records showing share of legal heirs. This grants rights equal to that of owner to legal heirs. All legal heirs are at liberty to relinquish/release/transfer/gift/sell their share in anyone’s favor.
At some locations authorities may agree to accept even an affidavit about legal heirs…
The lender may even ask for succession certificate, and all legal heirs to be present to sign on sale deed, per internal procedure.
You may check with buyer/lender for their requirements as you may have to satisfy them..