How do I deal with the news of my father's second marriage, and what should I do about my feeling of injustice towards my mother, and the impact of this marriage on me and my siblings?

It is the father's right to marry a second, third, and fourth wife, and his children are not permitted to prevent him or reproach him for it. He should choose a righteous wife, and he must deal justly among his wives in terms of expenditure, companionship, and lodging. Similarly, it is incumbent upon the children to honor their father and treat him well, and it is not permissible for them to demand that he divorce his second wife or their mother as long as he fulfills their rights.