What is the way to ease the conscience of a son who witnessed his mother's betrayal of his father as a child, and hates her because of her actions, knowing that she still communicates with some of those with whom she betrayed her husband, and he cannot tell his father for fear of family dispersion? And is he sinful for hating his mother for her deed?

You must conceal your mother's sin and not tell anyone what you saw her commit. Your statement that religion obliges her to confess this to her husband is incorrect. Rather, she must conceal herself and repent to her Lord, for the Prophet, peace and blessings be upon him, said: "Avoid these abominations that Allah Almighty has forbidden. Whoever commits any of them, let him conceal himself with Allah's concealment and repent to Allah. For whoever exposes his misdeeds to us, we will apply the Book of Allah against him." The Prophet, peace and blessings be upon him, also said: "O Hazzal, had you covered him with your garment, it would have been better for you." Your duty is to advise your mother to cut off her relationships with strange men without informing her that you know about her past actions. If you are unable to do so, then inform someone who has the authority to restrain her, such as your maternal uncles. If this does not work, then you must inform your father about this, so that he may stop her from associating with these people, but without informing him about the indecency she committed. A mother's committing of sins does not negate her right to dutifulness. If you fulfill her right of dutifulness and maintaining ties, then the aversion you feel towards her will not harm you.