When my wife and I got out of graduate school, her sister was about 19 and hadn't dated very much. She spent a lot of time at our house and went boating with us a great deal. I was skinny but I have a pretty big cock, (7 1/2'+) and a couple times on the boat I let it slip out of my trunks so she could get a good look at it.
She was amazed at it's size, and grabbed the head of it and squeezed it hard. After a while Mary wanted to play with it whenever we got the chance. She loved to rub the head of my dick and slowly stroke me. Once at the end of the day we were camping and found ourselves alone in the tent. My wife was down at the boat ramp so we had a few minutes to ourselves.
I took my swim trunks off and my cock sprung out at my sister-in-law. She smiled and grabbed it and started to stroke me, telling me what a big one I had. (I'm not sure how she knew, I think its the first one she had ever played with). I told her to go faster, and even jacked myself off a little to show her how. She started to beat me off faster and faster, and after a minute or two, I blew a huge load.
I shot the first spurt across the tent a good two or three feet, then a couple more on Mary's hand. She thought getting my cum on her hand was gross, but after she wiped it off she was OK. There were several mutual jack off sessions with my sister-in-law later on, until she got older. Now we are both married and have children, and never mention when we masturbated together, but it was great fun back then.