What is the ruling of Islam on a father who is able to provide but does not spend on his children and wife, preferring to spend on his parents and aunts, and then wishes to marry another woman, knowing that the older children are spending on their younger siblings?

A man is the maintainer and protector of his family due to what Allah has favored him with, and due to what he spends on them. He must spend on his wife, his young children, and his unmarried daughter. If he falls short in this, advise him gently, seek help from people of knowledge and righteousness, and be patient with him. If he does not respond, it is permissible to refer the matter to the Sharia court to compel him, and this is not an act of disobedience. As for your brother, he is not obligated to spend as long as the father is alive and capable. However, if he does, it is good. And if the father asks him for help, obedience is due to him in what is good. The father's marriage to another woman is permissible if he can be just and provide for her. It is obligatory to treat the father well, even if he falls short, as this does not justify disobeying him.