Is it permissible for a mother to ask her son to make his sister's share of the inheritance equal to his instead of half, and is the son's agreement to this request an act of disobedience while seeking to please his mother, and is this request from the mother شرعي (Islamically lawful)?

It is not permissible for a mother to ask her son to make his sister's share of the inheritance equal to his own, and there is no obedience to her in that, because obedience is only in what is right and lawful. However, if she advises him to give his sister something from his inheritance, this is permissible, and obeying the mother in this is commendable if the son donates with a willing heart.