Is it sinful to help the wife's father with a will that deprives the son of his legitimate inheritance in exchange for guaranteeing him an income?

This action is not permissible because it is a circumvention to give a share of the inheritance as a bequest, and the Prophet, peace and blessings be upon him, forbade that. However, if a person wishes to benefit his child, he can endow some of his wealth to those among them who are in need, and appoint the most trustworthy of them as an overseer of this wealth so that the deserving one receives his right without transgressing the limits of Allah.