Is a gift valid with only one-third, and is the father obligated to sell the apartment to his son so that his sister does not have a share in it?

If a father wishes to transfer ownership of an apartment to his son during his lifetime, this is a gift (hibah) which necessitates justice among the children. It is not permissible to favor some of them unless there is a legitimate reason, such as a need for housing. However, if the transfer of ownership is to occur after the father's death, this is a bequest (wasiyyah) to an heir, which is not enforceable unless all adult, mature heirs agree, due to the Prophet's (peace be upon him) saying: "Indeed, Allah has given every rightful person his right, so there is no bequest to an heir." The scholars have unanimously agreed that a bequest to an heir is not valid unless the rest of the heirs approve.