The land your father left, as well as what it yields, is the inheritance of all his heirs. Like other items of the estate, the land and its yield must be divided among the heirs from the moment the deceased passed away. There is no harm in buying your sisters' share of the land, even before dividing it, provided (Part No. 16; Page No. 461) that their shares are clearly predefined. You must pay them their share of the rent from the land before buying their shares of the estate, unless they waive it. May Allah grant us success. May peace and blessings be upon our Prophet, his family and Companions.