Is Zakat obligatory on a house we inherited from our father and have been renting out for 25 years, and if so, how much is it? Also, should the inheritance of our sisters be calculated based on the land's price at the time of death or its current price?

Zakat is not obligatory on the value of the house. The rent of the house is to be divided among the heirs. Whoever's share reaches the nisab (minimum threshold) and a hawl (full lunar year) passes over it, must pay zakat on it. The daughters own their legal share of the house, and if the house is sold, their share of the sale price should be given to them. Caution must be exercised against preventing daughters from inheriting or diminishing their legal right.