Deemed Conveyance - Transfer of Property Card

The conveyance of the cooperative housing society has not been executed. The society was registered in 1974. Property card still has owners name & was not transferred to builder & in turn has not been transferred in the name of the society. The owner has passed away. The society is now looking at executing deemed conveyance. There are 2 legal heirs - Son and Daughter both above 70 years present day, have 1 flat each in the society. Son is using his sisters flat & sister has not come for any society meeting for about 35 years. For deemed conveyance and transfer of property card, from the father of the Son and Daughter to the society which will be done present day viz., year 2019. Question: Is it correct when society says that only Son's signature, NOC is required to transfer property card in his fathers name to the society and no requirement for the sisters signature, NOC to execute above transfer because sisters address is not available & hence not reachable to communicate?