Legal heir certificate

Namaste ! I need some clarification and procedures for getting a Legal heir certificate for my mother, based on her deceased father (my grandfather) . She had received an house, from her father, through Gift deed or settlement deed. My grandfather, my grandmother both have expired back 20 years ago. My grandfather had 5 sons and 1 daughter (my mom). All my 5 uncles have also deceased and the last and only surviving heir is my mother. My uncles family or their heirs are not willing or interested to prove/authenticate my mother in getting the legal heir certificate. Now, how do I proceed further, to get the legal heir certificate, for my mother based on her father name. I have my grandfather's death certificate true copy and photocopy of the gift or settlement deed.I wanted to get this certificate, since , it will be required in future, to prove the ownership or right of the house, that my mother owns now. Also she is crossing around 70 years of age now. Kindly guide me with your experiences.