I may have picked the wrong cousin?

I slept with a guy a few years back we did not date or get in a relationship end up he just wanted sex but his cousin is who I always actually wanted to date. Fast forward now the cousin wants us to try again be serious and date. I have been stand off ish because of sleeping with his cousin and feeling like I can’t date him now. Should I tell him hey I slept with your cousin or not date him at all? I do wanna try again but I do not know. Is it his business? Or should I just date him. This was probably 3 years ago.

I may have picked the wrong cousin?
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