This happened many years ago when I was 15 and is still very memorable.
My sister Anne is two years younger than me and at the time she would have been about 13 getting on for 14 and I was 15. As a young teenager I jacked off at every opportunity and I seemed to have a hardon all the time although I guess that's not unusual for a young teenager. Occasionally we were left at home alone especially during the holidays, we had done the normal brother and sister if you show me yours I'll show you mine, I would always be rock hard in anticipation of her lifting her dress and taking her knickers off and would have to jack off as soon as I could after my sister had gone. I only jacked off in front of her twice, the first time my sister had taken her knickers off and was showing me her pussy, I'd dropped my pants and was stood in front of her rock hard and although I'd never jacked off in front of her before I started stroking my cock, I said that it felt good when I did that and that if I carried on I'd cum. I don't think she really understood what that meant or if she did she didn't let on but I continued stroking my cock, it only took a short time and I was cumming.
A few weeks later she asked if I would show her what I had done again but asked if her friend, Alison, also 13, could watch as well, my heart was pounding but I agreed although I was not sure if she really meant it, she had obviously been talking to her friend about it, her friend was an only child so had probably never seen a cock before. It was a few weeks later during the school holidays that we had the opportunity to do this, our parents had gone to work and my sister and myself were alone in the house and knew that our parents were out for the day so there was no chance of getting caught. I asked my sister if she wanted to watch me again and she said yes so I said that if her friend Alison wanted to watch that she should go and ask if she could come over to our house. Whilst I waited my heart was pounding and my cock was already hard in anticipation, I still wasn't sure that we would actually do this.
A few minutes later my sister returned with Alison so I said that I would go up to my room and that I'd call them when I was ready. I took all of my clothes off and got in to bed and for some reason put a sock over my cock to hide it from them. I called downstairs and said that I was ready and that they should come up, the door opened and I was in bed, I think they expected me to be stood naked in front of them but I said that I was naked under the covers and that they would have to pull the covers back if they wanted to see my cock, they both grabbed the covers and I half heartedly tried to stop them but finally let them pull the covers back and although I was naked my cock was covered by the sock. My sister said that I had agreed to show them everything and I said no, I think that they thought that I was chickening out so they both started grabbing at the sock which was hiding my erect cock, feeling their hands grabbing at the sock which my cock was hiding in was an unexpected bonus and pleasure and it didn't take long and the sock was gone and I was completely naked in front of my sister and her friend. For what seemed like a long time they looked at me and I looked at them none of us quite sure how to proceed.
I thought that there was no point in turning back now so I lay down on the bed and started stoking my cock, both of them watching me. It would have been great for them to do it for me but I wasn't too bothered, I had my sister and her friend both watching me stroke my cock and it didn't take too long before I knew I was going to cum and told them to watch closely, my cock was pointing straight up and my cum went in the air and then on to my stomach, they were both still watching as my cock shrank back down, that was the end of the show and sadly it never happened again.