This happened when I was 16. During the school holidays I used to stay with my sister June and her husband (Andy). My sister was 23 and married to this absolutely gorgeous 24-year-old. I used to really enjoy visiting with them because their room was next to mine and when they made love it was quite noisy and I could masturbate without having to worry too much about the noise I was making.
One night I had just been to the toilet and as I was walking past their bedroom I heard the sound of sex and noticed that the door was slightly open so I looked in through the gap. What I saw made my clit jump and tingle and the juices ran so freely and so quickly I thought I was wetting myself and about to cum at the same time. There was June lying on her back masturbating while Andy was naked and taking a video of her.
My hand immediately went to my pussy and I started rubbing my labia. I was careful not to touch my clit (although I was craving to) because I was afraid that I would make a noise and be found out. I was mesmerized by watching June masturbate but also craved to see Andy move so that I could see his penis. As he was videoing June he moved around the bed and suddenly his erect penis was in full view. I could hardly contain myself when he then grasped hold of his boner and started to stroke it. At this point I instinctively rubbed my clit, let out a moan and fell against the door. Boy was I embarrassed but also incredibly turned on, horny and close to cumming.
Just when I thought that I had blown it Andy invited me to join June and himself. I jumped at the opportunity. They both said that they used to enjoy getting off knowing that I was masturbating in the next room (they could hear me) and now they wanted to watch. I lay on the bed next to June and we both continued our masturbating with Andy videoing us while he stroked is cock. I came first with the most powerful and prolonged orgasm I had ever had then June came even harder and squirted out some fluid when she came. I was then fully focused on Andy but he stopped stroking himself and asked if I would video him and June while June masturbated him. I was hoping like mad to see him finish himself off but was more than happy with his plan.
I was sooo excited that I could hardly keep the camera still. Not long after June began stroking Andy she began to masturbate again. They were in a good position for me to capture them both. I couldn't take any more and began fingering myself too. I noticed some fluid coming out of Andy's cock and thought that he may have been cumming (I had never seen an erect penis before let alone one cumming). At this point my sister began working on the two of them at a more intense pace. Andy was moaning and thrusting his hips in the air then he became very tense, let out a cry and shot semen all over the place. When this happened I came again and so did June.
When I went back to my bedroom I masturbated and came a couple of more times before going to sleep. The next day I went through their video collection until I found one of Andy by himself masturbating to full orgasm. That was the beginning of a very enjoyable holiday break. I have just cum again as I write and remember this.