How can one deal with a father who prioritizes his second wife in spending and treatment, and falls short in fulfilling the rights of his first wife and their children?

A husband must be just between his wives in terms of lodging and obligatory maintenance. He is not permitted to favor one over the other, as a severe warning has been issued for those who are not just between their wives. As for equality in non-obligatory expenses, such as gifts, it is commendable but not obligatory. If the father does not deal justly between his two wives or refrains from spending on his children, he is sinful. Nevertheless, the father's injustice or dereliction does not nullify his right to dutifulness and companionship with kindness. It is permissible to advise him, enjoin him to good, and forbid him from evil gently and politely, without harshness.