Should I tell my brother his girlfriend cheated years ago?

Anonymous

My brother's girlfriend had a drunken ONS way back in 2018. Things have gotten serious and he's thinking about forming a family with her but I still feel guilt.

All of you must be now wondering why I haven't told him about her ONS and why I'm covering up for her. Well it's because that guy she cheated with was me. We were both just as responsible that night (she actually made the moves first and I reciprocated; we were both drunk) and promised it would never happened again. She was remorseful and so was I. So we made a deal to never say a word about it. I haven't gotten drunk since.

Though I really thought that at some point my brother would break-up and nope, their relationship has apparently gotten stronger. Still should I tell him or not? Or can people make one bad decision (a terrible action) and learn from it like I did.

Updates
11 mo
No one knows about it except us two. If this gets out by either of us speaking, it won't be just her reputation going down, I'm screwed too.
Should I tell my brother his girlfriend cheated years ago?
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