Late Night Revelations With My Ex's Sister

So, here I am, about to confess something that's been eating at me for weeks now. I'm in my fifties and just got out of a relationship with someone who was almost twenty years younger than me. We were really into each other but it ended up not working out, mostly because of some pretty messed-up stuff on my end. I'll start from the beginning: My ex's sister had always been this kind of shy girl, maybe in her mid-thirties by now, who used to come around when we visited his family. She was so quiet and reserved compared to everyone else that I never really paid much attention to her before... until the night it happened. I'd just broken up with my ex after a big fight where I realized we were too different. My friend's house was right next door, and as I sat alone in the dark thinking everything through, she came over unannounced. She said she needed someone to talk to after overhearing about our breakup and seeing me crying. The moment she walked in with tears in her eyes, something changed inside of me, maybe because I felt sorry for her, or maybe it was just the loneliness talking. We ended up sitting there for hours, and before I knew it, we were kissing passionately on his living room couch. She seemed to like it as much as I did, which only made everything more confusing. The sex that followed wasn't anything remotely subtle either, I mean, every part of her body was so soft against mine, the way she moaned and arched into me... we went at it hard on his couch without any concern for what might happen next. It felt both wrong and incredibly right somehow. I'm still trying to figure out if that night meant anything more than just a momentary release for us both, given her brother was my ex. But every time I think about how our lips met and hands explored each other's naked skin that night, the guilt only grows stronger. Now here we are months later, and it still feels so surreal, like one of those dreams you can't shake even when you wake up.