What is the legal ruling regarding the questioner's confrontation of his mother and informing his father of what he saw, and hitting his mother? Is there expiation due for that? Is he responsible for the divorce of his parents? What is the ruling on his weak feelings towards his mother? How can his mother expiate for her deed, and is her divorce sufficient for that?

If what you mentioned about your mother and what she did is true, then she committed a grave evil by bringing a stranger into her husband's house with ill intent. This cannot be expiated by absolving her responsibility through divorce. Rather, she must repent and seek forgiveness from your father, unless she fears a greater harm.

As for you, you erred and did wrong by informing your aunt, exposing your mother, and hitting her. This is an act of disobedience (to parents), and your duty is to repent and ask for her forgiveness.

It appears that you bear no sin for the divorce, and there is no blame for the weakness of your feelings towards your mother, as these are matters of the heart. However, beware lest this lead you to disobedience.

As for your father, part of your dutifulness towards him is to strive for his betterment through prayer and sincere advice, and to involve virtuous people to influence him if necessary.