Several weeks ago my wife was out of town and my sister-in-law called to offer me dinner. She divorced about five years ago and has a ten-year-old son. I accepted and went to her house. When I arrived, her son was lying on the living room floor watching a cartoon type movie. She fixed me a plate and I sat on a chair in the living room to eat and watch the movie with her son.
She had already eaten and was sitting on the couch wearing a t-shirt and a pair of loose fitting shorts. While I was eating, I glanced over at her and noticed her left hand was between her legs. I didn't think much of it at the time and continued to eat and watch the movie. A few minutes later I glanced at her again and this time I realised she was slowly working her clit through her shorts with one hand and her other hand was at the side of her face blocking my line of sight to her eyes. I watched her working herself for a few seconds and realised she had her eyes glued to my crotch the whole time.
This started the rise in my jeans and soon I was rock hard. I glanced over at her again and she was working herself a little harder but still not really obvious. I decided it was time to really heat her up so I started very slowly tensing and relaxing my erection so it would move the now quite large bulge in my jeans. I continued this for a few minutes. All the while watching the movie and eating the dinner she had prepared. I could hear her breathing get faster and deeper.
I glanced over again and noticed her shorts were a little wet where her finger was working her clit. At that moment, she moved the hand that was blocking our eye contact and she saw that I was watching her. She quickly jerked her hand back between our eyes but continued working herself for a few seconds.
Then, I guess she realised she was caught and she stopped. I finished my dinner and watched the rest of the movie with her son. We exchanged some small talk, I thanked her for the dinner and I went home and wanked several times to the memory of what just happened and I'm sure she finished herself off also.
We have never discussed this and probably never will. But since then I have looked for opportunities to provide her with a view of my crotch and watched for her 'reaction'. Even when my wife and others are around. On one occasion I took it a little further but that's another story.