What is the ruling of Islamic law regarding the children from the first wife demanding their right to inheritance, given that the father has been living with the second wife for forty years and claims that the first wife has no right to the current properties?

It is not permissible for a father to favor some of his children over others, for the Prophet, peace and blessings be upon him, said: "Fear Allah and be just among your children." Scholars have differed regarding the ruling on the action of one who favors some of his children over others. The Hanbalis and the Zahiris held that this action is void, and their proof is the apparent meaning of the texts. As for the majority, they affirmed its validity, citing the action of Abu Bakr Al-Siddiq and the saying of the Prophet, peace and blessings be upon him, in the hadith of Nu'man: "Call someone other than me to witness this."

The most preponderant opinion is the former, because the Prophet, peace and blessings be upon him, called favoritism injustice and commanded its rejection, and whatever is unjust is void. Moreover, his saying, "Call someone other than me to witness this," was a reprimand and not a permission. And Abu Bakr's action does not contradict the Prophet's saying, peace and blessings be upon him, and he might have given his other children an equivalent amount. Favoritism leads to forbidden disobedience (to parents), so a parent must equalize among his children, either by returning what was favored, or by completing the share of the others.