Is it permissible to draw up sale and purchase contracts for apartments jointly owned by a husband and wife, so that full ownership devolves to one of them in the event of the other's death, given that there are no children, but there are siblings?

If the sale is a sham and the intent is for one spouse to take the other's share in the apartments after their death, then this is a bequest to an heir, which is forbidden if there are other heirs, due to the Prophet's (peace and blessings be upon him) saying: "Indeed, Allah Almighty has given every rightful person their right, so there is no bequest for an heir." Nevertheless, this forbidden bequest becomes valid and depends on the approval of the heirs.