Welcome back ladies and gentlemen of the internet. When someone smells a hint of betrayal or suspicion, it is a delicate dance to subdue the feeling. Everyone has insecurities, and everyone has their bad days; being in a partnership means talking through everything, seeing your partner with respect, and wanting to really listen to their perspective. In difficult situations, tact is essential to deescalate insecurities and rebuild trust, even if it wasn't actually broken.
In this Reddit thread, the original poster (OP) is a man whose wife accuses him of cheating on her with her sister. The context is that she has a new friend feeding her some potentially quixotic ideas that may not be predicated on truth. She sees that her husband and sister get lunch together, which sends her spiraling. Instead of calming her down and assuring her that there's nothing going on, he reacts defensively, leaves, and considers divorce. If he didn't do anything with his SIL, he's not doing a good job at showing it.