Family certificate

Dear sir My father has purchased a plot 20 years back and he expired 15 years back. He didn't left any will. We three siblings are in agreement to transfer the property in my mother's name who happens to be the equal share holder of the property. We were advised to make a relinquishment document in the name of my mother to transfer our share in ours mother name. Till this point every thing is ok. But when I called up the registered office We were told that a family certificate has to be produced along with the release deed. When we applied for family certificate the Revenue office has denied saying that it should have been applied with in the year of my father's demise. Further he has advised that the matter has to be filed in the court to get a family certificate. My father was a retired railway employee and my mother is recipient of the widow pension. We three silblings are gainfully employed and settled with our spouses at three different places. What all documents need to be produced to acquire the family certificate from the court may be advised. Thanking you sir