Caught by college friend..
Don't know if this is as much embarrasing as it is giving someone ammunition for later use. When I was in college, my hot older sister (whom I've just confessed messing around sexually with) would visit me some weekends, as she was also a grad of the same school.
During the Fall, she'd wear tight, ass-clinging jeans that formed to her body so well I couldn't keep my hands off of her. Sweaters or shirts that showed off her ample b****, hair worn down, long nails polished red (the school's color, and very sexy), and usually alone, I'll be honest: she made me look good.
Anyway, we were no strangers to flirting and physial playing around, and one weekend visit, she came by dressed as described, revving up my hands and desire for her body. We were in mid-hug in my dorm room, and I groped her ass, having at it with both hands as we chatted. She laughed and kept jokingly saying "St-op!".
My friend Ryan, who, though a nice guy, was a bit of a prude, stopped by to chat, noticed my hands having at hot sister's ass. Ryan also knew who she was, so there was no lying about this being a new girl I was with or dating.
He said nothing until later that evening, after she'd left. "Dude..Were you grabbing your sister's ass? That's sick!" I remember him saying. "I mean, she's your sister! And you were grabbing her ass with both hands!". Also shook his head, repeating "Sister..It's your sister!"
Told him yes, admitted to it all. From then on, we remained friends, but I always knew he knew, and could use it whenever he wanted. Thankfully he didn't.