When we were younger, my sister and I shared a room, with our beds on opposite walls. She is three years younger than me, and we both had pubic hair when this started. One night I had placed my dick and balls between my legs so that they could not be seen from the front. Then, I called my sister over to my bed and threw back the covers. Now, she knew that I had a dick and balls, but when she saw none, she looked on in silence. After a short time I parted my legs and my dick popped out with a 'sproing', and she giggled out loud. I covered quickly, in case our parents came in to check on her giggling. When they didn't she pulled the cover back off of me. Nothing more happened and she went back to bed.
A couple of nights later I thought I would be bold and I pulled my pjs and briefs down and walked over to her bedside. She opened her eyes and looked at me as I pulled her cover down and then her pjs and panties. We both just stared at each other without saying anything.
Similar activities happened over the next few weeks, until one night she grabbed my cock. Damn, it felt great for someone besides myself to be holding it. She then opened her legs and gave me my first view of her pink pussy. It looked wet. Then, it felt wet.
Finally, one night I got an idea, and I laid on her bed, head to toe fashion, facing each other. I put my cock between her legs, against her pussy, but sticking out under her butt. That felt hot, like it should be there. We laid there motionless, when I felt my balls swell and my dick getting even harder. I tried as mightily as I could, but I could not hold it back.
First, there was a preliminary salvo, when she asked if I peed on her. I said, 'No,' but remained in place. Then, the rest of it happened. I just squirted and squirted and squirted, and she didn't move. To this day, some 50 years later, that is the most powerful orgasm I have ever had.
To close the story, she still said that I peed, and we got toilet paper to clean her bed.