How should one deal with an unjust father who has disowned and slandered his children, taking into account the mother's oath not to visit or speak to him?

A father expelling his children and falsely accusing them is a grave mistake. However, his bad character and injustice do not negate his right to filial piety, nor do they permit his abandonment. Allah has commanded companionship with kindness towards a polytheist parent. The mother's right is greater, but it is not permissible to obey her in an act of disobedience, such as her command to abandon the father. You can maintain ties with your father without your mother's knowledge. Filial piety is among the greatest causes of Allah's pleasure, and disobedience to parents is among the greatest causes of His displeasure.