I was away with my brother and sister in law at a cabin in South Carolina. My wife couldn't make the trip because of her job.
The morning that we were to leave, my brother in law, Jim got up early to get everything ready for our trip home. After he went outside I just talked with my sister in law Liza. We all just slept in our underwear in different bunks. These cabins were very small, maybe 12 by 12 or so. Liza said she was going to take a shower and got out of the bunk below me. She was wearing just a t-shirt and pink panties.
She stretched after she got out of the bunk, making her hot-pink panties very visible to me. At the same time I had to pee really bad so I asked her to let me use the bathroom first. I jumped down just in my underwear. I had a pee hard on which was quite visible through my white briefs. I walked in front of her to use the bathroom and I noticed her staring at my hard on. I used the bathroom and came out, while Liza was looking for Jim's keys. She wanted me to help her look. So here we were; me in my underwear and Liza in t-shirt and panties looking for Jim's keys.
Just being in such close quarters with my sister-in-law in just her panties was too much. I now had a major erection practically coming out of the top of my briefs. Liza noticed this and said 'What do you usually do about that?'. I was kind of embarrassed and said 'Usually after I pee or after I jack off, I don't have this problem,' which I couldnt believe I said! So she said 'Well why don't you go into the bathroom and take care of it or I can watch you take care of it.'
At this point I was not far from creaming in my briefs, so I went into the bathroom. She followed and said 'Do it in the sink.' I pull my underwear down and start jacking myself off while Liza watched me. It didn't take long before I shot my cum all over the mirrored wall. I had never shot my cum like that before. It was so exciting I was almost shaking. She said she'd clean up the mess when she took her shower. So I went outside to help Jim.
While I was outside helping Jim I thought about his wife in the bathroom cleaning up my cum after having watched me masturbate. I became hard again and do to this day. Nothing has ever been said about it since.