Is a person held accountable for the hidden feelings of dislike for his father, despite being keen to obey him, due to his feeling of failure to defend his deceased mother against his father's continuous injustice, and the latter's treatment of his new wife differently from how he used to treat his mother?

It is obligatory for whoever is able to do so to repel injustice from the oppressed, based on the hadith: "Support your brother whether he is an oppressor or oppressed." Therefore, you must stop your father from his injustice through advice and reminding. If he does not respond, then bring in someone who has influence over him from among the people of good. Do not hate your father or despise him; rather, love him and treat him well, but do not obey him in disobedience to the Creator. Increase your supplications and charity for your mother, and seek Allah's forgiveness if you fell short in supporting her. Do not be angry at your father's kindness to his wife; rather, rejoice in it and do not let Satan find a way to cause corruption.