Is selling the current apartment and buying two apartments with its price—one for the son in his name, and the other for the three daughters in their names for the mother and father to live in until death, then it becomes theirs—considered a sin?

If the apartment is owned by the husband, the wife has no right to dispose of it except with his permission. As for her gifting it to her children during her lifetime, it is stipulated that there must be fairness among them, and fairness means equality between males and females in the gift. Gifting one apartment to a son and another apartment to three daughters is not fair, unless the daughters agree or if the son deserves preference, such as due to poverty. Suspending the effectiveness of the gift until death is not valid, and it becomes a bequest to an heir which is not enforceable unless the rest of the heirs approve.