Is an father's oath to deprive his daughters of inheritance, due to a disagreement with his cousin who is the father of his daughter's husband, considered an injustice? And is a father's prevention of his sons from sharing their sister's joy in her daughter's engagement considered an injustice? And is the sisters' complaining to their brother about their father's injustice considered backbiting?

Advising parents should be done with gentleness, tenderness, and a show of mercy, without argumentation or raising one's voice. This is achieved by being dutiful to them, lowering one's wing in humility, and speaking noble words. One can seek assistance from someone who influences him to advise him to repent and cease cutting off ties of kinship and depriving his daughters of inheritance.

If the father forbids attending his niece's wedding, one should try to persuade him. If he insists, and not attending would constitute a severing of kinship ties, then he should not be obeyed in that matter.

Your sisters' complaints to you about their father, seeking to have injustice removed from them and for you to advise the father to stop harming them, are permissible and not considered backbiting. However, if the intent is to cause division and stir up problems, then it is not permissible. You should advise them to endure their father's ill-treatment, be dutiful to him, and treat him kindly.