Will the mother be sinful if she registers an apartment in the name of her male son after the death of the father who had intended to do so, and without the approval of one of her daughters?

Mere intention is not considered a gift, and a gift is not complete until it is taken possession of. If the giver dies before the recipient takes possession of it, it reverts to the inheritance.

A will made by an heir is not valid unless approved by the rest of the adult, mature heirs, based on the Prophet's (peace be upon him) saying: "Indeed, Allah has given every rightful person his right; therefore, there is no will for an heir," and in another narration: "unless the heirs wish otherwise."

Accordingly, the son does not own the apartment due to the lack of conditions for a gift and a will. It is not permissible for the mother to register it in his name except with the approval of the mature heirs; otherwise, its registration is void.