The property left behind intestate by your grandfather shall be succeeded by his successors who are called as his legal heirs.
This is the law as per the provisions of Hindu succession act.
Though your father had been maintaining the property all along so far after the demise of his father, he cannot deny the legitimate and rightful share of his siblings who are co-sharers to the property.
As the property devolves equally on all the legal heirs it becomes the duty of all of them to maintain the property, however since your father alone had been maintaining the property till this date, he may render the accounts towards the maintenance expenses incurred by hi so far and may claim each individual co-sharer to settle the amount and take possession of their respective share in the property.
Alternately he may buy their shares or may even compensate them with a negotiated amount and get a registered release deed executed in his favor with regard to their share of property.