Family tree/genealogy document

My father bought a property in 1968. The owner had died before selling the property. When the property was sold his wife and children executed the sale deed and signed the document, having received the consideration amount, as per Islamic Law (Sharia). There was no existence of succession document at that time. Subsequently all the documents have been transferred to my father's name. Since then both my parents have died, the succession certificate has been issued by the authorities and the requisite documents have been transferred to children's name. Currently, we are planning to sell the property. Despite having all the documents, the buyer is insisting that we should get the genealogy document of the seller, which was not relevant in 1968 and besides 7 out of 9 members have died since. Is it true that if the property documents are older than 30 years the buyer should not insist on documents that were not prevalent 50 years ago. Kindly let me know what is the correct position. Is the family tree/genealogy document required now, as no property is sold or bought by anybody directly from them. We have all the documents including the succession certificate. Pl clarify. Thanks