OPP-Should He Tell Or Shut Up?

A man figured out that he knows his girlfriend's mom rather well. Should he tell his girlfriend or should he keep his mouth shut?

Mike & Kate-
I have been dating a woman for about 4 months now. She is 25 and I'm 32. She is great. I really like her. About a month ago I met her family at a BBQ. When I walked in the backyard I instantly recognized her mom. She and I hooked up about 2 years ago right after her divorce. We met in a bar. She looks amazing for being a mom. She is only 45 or was at the time. I found the whole cougar thing hot and what they say about recently divorced woman is true. They are wild. Anyways, it was just a sex thing. We slept together only a couple times. We both understood what we were getting from the other person.
Fast forward and now I'm dating her daughter. I really click with this girl. I could see us getting serious. She has invited me to Thanksgiving dinner at her mom's house. If I go it's going to be so awkward. I'm worried I will give away our secret by how uncomfortable I am or what if her mom has too much wine and says something? I doubt my girlfriend would understand if I say I can't go. My family doesn't live in Idaho and I have to work on Black Friday so I have no good excuse. Do I come clean and tell her about sleeping with her mom. It meant nothing to me. Or do I keep my mouth shut and if I do that, how do I get over the awkwardness?

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