When I was 19 I lived with my half brother and worked on his farm for a while. He was a lot older than me and he had a son who 13. I was sleeping in my nephew's room. In a separate bed, of course. We were sleeping bare ass because of the heat.
Pretty soon after the lights were out I started hearing him shaking in the sheets and I knew he was beating his meat. So I started doing it too. We would beat off every night. We both knew the other one was doing it. But we never talked about it.
One night I got too horny. I got out of my bed and crawled in bed with my nephew. I got myself behind him where my cock was between his legs. Then I reached around and took his cock in my hand. I humped his leg and stroked his cock at the same time. I came between his legs and he came in my hand.
When I got back in my bed I was sorry about what I did, I was worried he would tell my half brother and I would be in big trouble. But nothing happened. So after about a week I got back in bed with him again and we did the same thing.
From then on I was crawling in bed with my nephew almost every night for a jerk off. He always got hard for me and he started taking my cock in his fist when I took his. So he must have liked it, I know I did. We never talked about it though.
This went on for about 6 or 7 months, then I went in the army.