Hi Reddit,
I’m in a really confusing and emotionally draining situation right now, and I don’t know how to handle it. I’m hoping for some outside perspective.

Earlier today, my sister’s ex-boyfriend called me. He told me that they had been together for 7–8 years. I knew about the relationship, but I assumed they had broken up long ago. He said they only actually broke up in December 2024, and that for about 1.5 years before the breakup, she had been cheating on him.

He says he didn’t confront her earlier because he wanted to see how long she’d continue lying to him. He claims she often tells our family that she’s going to the library but instead goes to meet other guys—sometimes at hotels or private flats. He said she’s been involved casually with 4–5 guys during that time, and that many of them may have taken advantage of her. He even said that he contacted some of them and that a few have her nudes, though I don’t know whether those were consensual or not.

He also said that my mom knew about all of this. Apparently, he used to talk to her often, and she never told me anything. He believes that my mom and sister both have secrets on each other, which is why they’re protecting one another. He hinted that my sister might know something about my mom’s personal life (possibly an affair), though I personally don’t believe that.

My dad works in another city and only visits on weekends. My parents have been together for 28 years and seem happy, so this is all very hard to process.

Right now, I’m just stuck.
How should I approach my sister about what her ex told me?
Should I talk to my mom or hold off until I know more?
And how do I manage this emotionally without blowing up my family?
I really need some help navigating this without making things worse.

Thanks for reading.