My wife’s younger sister has always been a flirt but nothing ever happened, but compared to her sister, my wife, I got the best deal! At a family bbq some years ago though I was horny and took an opportunity wearing shorts ( I always cut the netting out) to sit on a deck chair opposite where I knew she would likely sit on the ground.
Carefully positioned my cock near the trouser leg and having a plate on my lap only someone sat where she would sit could see up my trouser leg. She did sit down and within a few seconds her eyes kept moving to my ‘accidental’ exposure.
She looked 20 or 30 times during that meal and I got a great thrill knowing she had seen me.
If someone else had sat there I could easily have closed my knees
Very low key but meant a good hard session with the wife in her parents spare room that night. Still think of that afternoon.