When I was 17 my sister and I were talking about sex one day. She asked me if I ever beat off. I told her I did. She asked me how often and I told her I probably did it once a day or at least every other day. She told me she had heard that guys did it a lot.
She gave me a look and said 'could I watch you?' I asked her if she were serious and she said she definitely was serious. She said 'I just want to see you do it.' Our parents were at work and we had about two hours before they both came home. I said 'under one condition ... you need to get naked with me.'
She told me she would, so we found ourselves undressing together for the first time since we were about five years old. My sister was now 16. When I got down to my underwear, my rod was really hard and quite obvious. She was in her panties and bra by that point. I said 'here goes!' I pulled my underpants down and kicked them off and stood naked in front of her. My dick was wildly hard. Her eyes glued to my dick as she slid out of her panties and removed her bra.
I had never had any romantic feelings for my sister, but had she not been my sister I'd have fucked her brains out right then and there. Her body was wonderful! Women didn't shave much back then and she had a thick pelt of pubic hair. Her C cup tits and brown nipples were firm and full.
She asked me if she could touch my dick and I let her explore my dick with her hands. It felt good. She let me feel and caress her breasts, and my hand explored her hairy pussy. She again asked me to jack off for her. I asked her to lay down and spread her legs so I could look at her pussy while I did it. She opened really wide for me and I kneeled between her legs and slowly jacked my dick. She asked me a bunch of questions as I did it, like how hard do you hold it, is it better to stroke fast or slow, etc. I believe she was wanting to beat her new boyfriend off but she had never done it and she wanted to be sure she knew what she was doing when she eventually did it to him.
The whole time I jacked she never really touched herself. She just showed me her wide open pussy. I guess I really didn't know women masturbated then. Of course, I knew a few years later and I wish I had known when we did that because I would have asked her to do it with me. Anyway, I jacked until I was about to cum and I told her I was almost there. She was watching so carefully, then I spurted gobs of cum out all over her stomach. 'Ooooh ... ICK!!! GROSS!' she was saying. 'EEEH ... get that stuff off of me!' She didn't move a muscle as I got some toilet tissue and wiped it off of her. I smile when I remember that.
That was my only experience with my sister. Nothing ever happened again. I'm sure that she overcame her squeemishness about men's cum in the years following. It was comical though looking back on that day. I also still masturbate now and then as I think about that experience with her.