Hello,
1) In the absence of a Will after the death of your grandfather all his children would have equal right to the property. In other words ideally the property should have been divided into 5 equal parts.
2) If the grandfather during his life time had transferred the title of the property by a gift deed or any other form of transfer while deciding on division this fact can can be considered. In the absence of any such transfer the entire property will have to be divided into 5 parts.
3) If the eldest daughter has passed away her legal heirs can claim right on property that their mother would have inherited.As I said earlier even if the eldest daughter registered the house in her name that house too must become subject of division in the absence of document to prove how the property was transferred to her in the first place.
4) Regarding the uncle who has been missing from action for 40 plus years, a publication in leading news papers will have to be made, calling for response from possible level heirs of any.
5) Mere occupation and payment of taxes will not give title to property. Therefore 40 years of tax payment is not advantageous.
6) File a partition suit after talking serious regarding the missing uncle.