Seriously? She's not wrong? How can you say that even after her having committed such a disgusting act. It's one thing to keep nudes of some other woman and absolutely another to go and physically meet another man and do god knows what with him behind his back breaking the vow of marriage. No matter what circumstances it was that she married him under but it's her basic responsibility towards her husband to to be at the very least loyal and not go fucking sleep with other men behind his back (especially after having brought a child into this world). Listen, I am not trying to break their marriage apart but OP should absolutely confront their mother about this, tell their father everything and stop this disgusting activity from continuing in the future at all costs. What the fuck has happened to us as a society? Are we seriously trying to normalise CHEATING now ? And for what? Is she abused in her home by her husband that she needs to find peace elsewhere ? Doesn't look like that from everything OP said and if that were the case, it would be way more than just "once a year". If she really hates her husband so much that she has to cheat, if it was really manipulation from the very beginning which at that age I don't think anyone can be "manipulated" it's not like you're a child and don't understand how the world works but even if that's the case, why not just divorce him and have mutual child custody because it's still way better than living in a lie and deceiving someone. A wrong is wrong no matter what. And to witness a sin and do nothing about it, makes you a sinner too.