If all the family members (who are alive and adult) are owning nisab (prescribed amount of wealth for zakah) then one share will not suffice for all, but everyone will have to offer separate qurbani. And, if none of the family members owns nisab then no body is required to offer qurbani. Qurbani can be offered from those who are already dead. In case of wasiyyah (will), the heirs are asked to offer qurbani from the one third wealth of the dead, but if the dead left no wasiyyah then the heirs have option whether they choose to offer qurbani or not. In case there is no wasiyyah, it is not prohibited to offer qurbani with the intention isaal sawab (delivery of reward) to the dead, but it is necessary to offer qurbani first from those family members on whom qurbani is wajib.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband