Release of rights over ancestral land for registration

The ancestral agri lands have patta and other documents on grandmother and her 3 sons. Grandmother and elder son are no more. The agri lands have been earlier partitioned but not registered. Now the divided individual lands is to get registered on elder son wife, middlest son and younger. So now as a son(grandson) of elder most have to provide consent by signing while registrations. But as could not appear for registration in person due to circumstances, what kind of document/certificate as of noc could be provided so that the registration process can bone smoothly.