Is it permissible for the father's wife to monopolize my deceased father's apartment with a 20-year lease agreement, knowing that my father had gifted her the apartment then retracted the gift before his death? And are we, the heirs, obliged to execute this contract, or do we have the right to claim the apartment?

What the father issued towards his wife is either a gift or a sale. If it was a gift and the apartment was not taken possession of, then the donor can retract it according to the majority of scholars. As for the Malikis, retraction is not permissible according to them except in what a father gifts to his child. If it was a sale, then the sale is binding on both parties, and neither of them has the right to annul it. It is not permissible for the father to use stratagems to invalidate his wife's right; rather, he must repent and hand over the apartment to her. As for the reconciliation that took place between them, it does not permit the father to own the apartment if he knew that his claim was false.