This happened 55-56years ago. Boys and girls did not reach puberty as young as they do now. There were only four of us in family, Mum and Dad and my twin sister and myself.
My twin sister arrived home from her summer holidays, our aunt and uncle came with her and stayed the night. Ann my sister was told to sleep in the spare bed in my room, as the visitors were going to sleep in Ann's bed. I was in bed before my sister and as it was summer-time and the weather was very warm, I choose to sleep naked. It was also easier to play with my 'my Mickey' and balls. Anyway, Ann came and got ready for bed, as we were only 14-15 at the time and I think we had just started puberty, as I remember hair had started to grow around the base of my 'Mickey'(that was what we called my penis back then). I remember Ann's breasts had started to develop, and were not very big. As soon as she got into bed I started to ask her about her holiday and all about our only cousin and how she got on with him. She went on to tell me how much fun he was. Tell me I asked? She went on to tell me about the funny things he could with his 'Mickey', going on to tell me that he could turn the tip of his Mickey inside out and he could make it produce some kind of milk. I told her that she was trying to make a fool of me, but she insisted that that was not the case.
With that she came to my bed, and pulled down the bed clothes only to reveal my throbbing boner. Getting hold of my throbbing cock, she said I'll show you. She started to pull back my foreskin which wouldn't go back for her. She kept trying. The more she tried to get it back, the more pleasurable the exercise was getting. When I said it was not possible to turn the tip of a 'Mickey' inside out she insisted and asked if she could continue for another little while? As I was enjoying myself so much I encouraged her to continue, BUT when I started to get strange feelings and did not know what was happening to me I asked her to stop. She insisted to give one final try with that she gave my foreskin one final tug. Then everything happened together. She did manage to turn the tip of my 'Mickey' inside out, and she also got that 'milk' to come out as well. OH the pleasure was such that the immense pain in exposing my knob for the first time was overcome by so much pleasure.
We were both amazed at our new discovery, and white sticky stuff was everywhere. By this time, my sister seemed to be very excited as she was making funny noises; her hand moved down to her crotch and started to rub herself between her legs. She seemed to get carried away as she proceeded to remove her knickers to reveal pussy, which, much to my amazement, had a little tuft of hair growing on top of her slit, which was glistening with moisture. She kept rubbing the top of her slit harder and harder until such time she collapsed on my spunk covered belly. I got a fright as I did not know what was happening to her. When I asked if she was all right, she just said it was just really wonderful feeling. It was such a good feeling that we agreed to try the same again. We were both amazed at my purple knob and the little pee hole. Over the years we had some wonderful times exploring our sexuality. We both married and our two families are nudist, and when the opportunity presents itself Ann and myself still enjoy a bit of mutual masturbation 55 years on.