Finding the main twist in 'Betrayal' really depends which one we're talking about—it's such a common title! Assuming it's that domestic thriller from a few years back where the wife discovers her husband's secrets, the rug pull for me wasn't that he was cheating. Everyone expects that.
It's that the 'other woman' wasn't a stranger or a mistress at all. She was his sister from a secret first family his parents forced him to abandon. The whole affair subplot was a red herring; the real betrayal was the lifelong cover-up by his entire family, and his wife realizing every happy memory was built on that hidden cruelty. The letters she found weren't love letters, they were childhood apologies. That shift from a marital drama to a generational tragedy is what stuck with me.
Made me look at my own family's polished stories a bit sideways, I'll admit.