One weekend when my parents were out, my older sister decided to organize a party in our house. I was 15 at the time, she was 19, almost 20. She invited one of her best friends who was coming in the morning to help her set all the things up and I helped a bit too, so I could finally join it (and wouldnÂ´t tell of course).
We had some argument that day so when she asked me to help her with some additional shopping I didnÂ´t go with her and stayed at home while she was about leaving for maybe an hour. Her girlfriend was having a nap upstairs and must have thought we both left. There is only mine and sister's room on the second floor of our huge house and I have to pass around sisters room to go to the bathroom, while my room is just facing the staircase. I think I went up very quietly with no extra intention and was going to the bathroom when I peeked into sisters room and saw this girl sitting at the computer, watching some porn with a hand inside her pants. Nothing was really happening and her hand was moving almost invisibly but it aroused me very much. I stayed there for a few minutes watching her, she was getting closer and closer to orgasm, I am almost sure she had one. At some point she turned her head towards the door and I left quickly, but she might have seen me. (Yes she saw, as I knew later.)
I didnÂ´t know what to do exactly, I was horny as hell but I didnÂ´t want to just go somewhere alone to jerk off. So I went to my room, let the door fully open, got completely naked, sat by the table and started to masturbate, nothing too loose. In a few minutes I heard her leaving sisters room and I was wondering if she would join me if she would find out what I was doing. Despite this pretty naive tactic she saw me right then and there and watched me for I while. I pretended I didnÂ´t see her and was jerking off until I cummed right onto my belly. She quickly disappeared.
Short after my sister went back and the other other girl was very reserved after our 'affair', she didnÂ´t even look at me.
The party was great, I saw some people making out here and there, even in my room once. When some people left and only about maybe ten were left, they started to tell dirty stories from their lives. Everybody sat in the circle and had to share a story. Some of them were real some of them were just fantasies, obviously. When it was my turn, my sister didnÂ´t want me to tell anything in this situation because I was five years younger than her and all the people around, but some other people kept saying I must contribute. They thought what I could experience yet and they were right I was virgin at the moment. So I told them that dayÂ´s true story, not using the real name but in every detail everybody knew that it really happened. It was arousing even for me and we all saw how sexy it was for all of us. Everybody was quiet for a while.
I was masturbating again that night and I heard my sister giggling with her friend until very late. When we met later next morning she told me I was a good guy and I told her I was making out with her that night very wildly (in my fantasy).