I have a twin who is a female. Yeah, fraternal twins can be of the opposite sex. Naturally, being twins has made us very close friends. We think alike and enjoy the same things. Sometimes we even read each other's minds.
We have always been very open with each other about everything. As little kids we used to show each other our privates when we played. That stopped by the time we were about 8. There were plenty of times over the years when we had to share a bed too, like when we had relatives visiting us and they would take over one of our bedrooms.
One weekend when we were 16 our aunt and uncle were visiting from out of town. They stayed in my sister's room that weekend, so she slept with me. When it was bedtime, I stripped to my undershorts and got in bed. It was no big deal at all. My sister had seen me in underpants a thousand times. She went into the bathroom and came out wearing a t-shirt and her boxer type sleeping shorts. We turned out the light.
We started talking in the dark before falling asleep. She said 'Dan, have you had sex yet?' I was floored. I was still a virgin though. I said 'no, have you?' She said she hadn't either. She said 'I wonder how it feels? It must feel good, but I wonder how it really feels?' I told her it probably did feel really good. She said 'I wonder though if it hurts when a guy puts his thing inside a woman the first time?' I told her I didn't know. Much probably depended on how big the guy is. She then said 'are you big?' I told her I didn't really know if I was big or not. She paused a while then said 'would you show me?'
I thought she was kidding at first but she wasn't. I said 'I guess so.' I turned the lamp on beside the bed, got up and locked the bedroom door, and laid back down on the bed. She was on her side with her elbow on the mattress and resting her chin in her hand with a look that was totally serious. I wasn't really even hard because I had never thought anything sexual with her. I slid out of my underpants but then, basically involuntarily, I started getting a hard-on. I let it go all the way hard and then I stood it up with my hand so she could see it well. She wanted to feel it so she held it and talked about it's hardness and size. She then asked me if I ever masturbated. I told her that I did. She asked if I would do it right then. I told her I would but she had to get naked too. She had no problem getting naked with me.
This was the first time I had seen her naked since we were kids. She had smallish but nice boobs and pretty pink nipples, and her pussy was covered in a mix of blondish brown hair. She let me look at her really well and touch her body. I then started jacking off for her. Every now and then she slid her fingers through her pussy but she wasn't really masturbating. She was very intrigued watching me jack off. She was really surprised when I shot ropes of cum all over my stomach as I leaned back.
We had a good time doing that. We never did it again though. We just went on like it never happened.