What is the method for distributing the estate of a mother, which consists of one son, a husband, two grandchildren, and siblings, taking into account her desire to divide her wealth between her son and her two grandchildren, and is she permitted to distribute one-third of her estate as she wishes?

The question encompasses several points:

Firstly: The heirs in this case are the husband (who receives one-fourth due to the presence of a son) and the son (who receives the remainder of the estate by ta’sib [residiary heirship]), while the brothers and sisters are excluded by the son.

Secondly: A bequest to an heir is not permissible, based on the Prophet’s (peace be upon him) saying: “Indeed, Allah has given every rightful person his right, so there is no bequest for an heir,” unless the heirs permit it.

Thirdly: A bequest is considered valid upon death. If your mother bequeathed something to her two grandchildren (your children) or to her brothers and sisters, it is permissible as long as they have no right to inheritance, and provided the bequest does not exceed one-third. As for a gift given during one's lifetime, it is considered a donation.