You can never really know what’s going on in someone else’s relationship, no matter how close you are to them. In a lengthy series of posts by u/throwawayAITA-SISTER, OP revealed that she had discovered that her older sister was having an affair and debated telling her BIL.
She said, “I was shocked because my sister and her husband are the epitome for love that never dies. Literally. They have been together for 15 years and they still look so much in love, the way they treat each other. I felt anger and despair because I love BIL as a big brother and he has been there for my sister when she been her worst mentally. He was there taking care of her without ever once complaining.”
She tried to confront her sister about it, but didn’t notice the red flags even though they were right there in front of her. She said, “Please don’t hurt us, you will be hurting my husband more that you hurt me.” A few days later, she revealed that she and her “perfect husband” hadn’t slept together in 12 years because he was impotent and the only reason why she is better mentally not is because of her affair partner, whom she met through work.
“Yesterday she asked me to meet her. She was with him, the audacity. She looked totally broken and he looked smug and angry and he didn’t even want to look at me. She asked me to think of all the people that would be hurt if this came out and she said that they will end their affair and promised to never do it again. He got upset and said ‘Or we could just tell everyone and stop hiding’. She got angry and told him to shut up.”
“I don’t know what to do. She said that she kept her husband’s secret because he is embarrassed and he is very possessive and jealous (I have noticed he is the jealous type) so he didn’t agree to open marriage. She swore it was over with this other guy too and showed me texts where they had huge fight because he actually thinks I should tell her husband so they could be together but I don’t know.”
This wasn’t the end of the story, though, and it was revealed through comments and updates that OP’s BIL showed signs of abusing her sister. She was just hyperfocused on the cheating aspect and made things worse by telling her BIL about the cheating.
In a final update, u/throwawayAITA-SISTER said her sister and BIL moved back to his hometown to allegedly “work on their marriage” but not long after, her sister ran away in the middle of the night. It’s sadly unsurprising that her BIL was being abusive and forced her to quit her job, was withholding her electronics from her, and more.
All that is a stark reminder that it’s never just a slap. You never know what someone else is going through, and your “good intentions” can cause real harm.