In my early teens I was desperate to see any girl naked, so it wasn't that unusual that I fantasized about seeing my little sister naked. She was two years younger than me and attractive enough that I noticed her developing curves despite myself. Altogether from about 13-15 (me not her) I would guess I would think about catching my sister naked about five percent of the time I masturbated. I didn't think of her as a proper girl and never went further in my thoughts than seeing her nude.
Things changed dramatically however, one day when I was fifteen. I'd come home early and was jerking off, thinking about a girl at school with a very sexy ass, when, in my mind, she morphed into my sister (who even at thirteen had a great behind). Ordinarily I would have pushed the notion from my head, but for some reason I decided to experiment and started to imagine 'doing' my sister. At first it felt a bit nasty and not at all sexy. But as I got into it the perversion started to turn me on and by the time I came I realized that this was the best wank I'd ever had!
From then on my sister ruled my fantasy life. The thrill I got from mentally committing incest with her put her streets ahead of any other girl. She went from being in five percent of my fantasies to ninety-five percent! Strangely, a side effect of this was that the only other girls I ever imagined were my sister's friends.
On a day to day basis, nothing much changed in the way I interacted with my sister, except for me admiring her hot little body when I thought no one would notice, and I congratulated myself on handling things so well. Part of my 'coping mechanism' was to get home from school long before my sister and jerk off. I'd toyed with the idea of letting her catch me, but thought the risks of her telling our parents (or worse: the other kids at school) weren't worth it. That is until one day I got home and was just getting started when I heard the front door open and my sister and her friend Karen's voices carry up the stairs.
My dick was doing all my thinking at the time and in a split second decided that these two hot girls were going to get their first look at hard cock. I jumped up from my bed, switched my main room light on and, at the same time, pushed my room door open half way. The landing was quite dark, so anyone coming up the stairs would now be able to see into my room without being seen themselves. I also turned on my radio so that they'd think I hadn't heard them come in. Finally, I went over to my drawers and opened one, standing with my back to the door.
Even above the music I could here their footfalls as they climbed the stairs and my heart pounded with anticipation. As they got to the top the sound of their steps suddenly stopped and I knew that they had to have spotted me and were now staring at my bare ass. Making an effort to stop myself shaking, I rummaged in the drawer as though looking for something and after a few seconds pulled out a t-shirt and threw it on the bed. I then removed the one I was wearing and turned round, giving them their first view of my boner, and headed for the bed.
I hadn't consciously planned what I'd do next, but when I got to the bed my dick felt enormous and the idea of giving them a show suddenly seemed right. As I sat down, out of the corner of my eye, I caught a slight movement and looking indirectly saw that my room door was slowly opening as though pushed by a light breeze. Turned on by my sister and Karen's apparent eagerness to see more I leaned back, blocking my line of sight to the door, and slowly started to massage my cock.
I used my left hand to steady the base and wanked with my right, much more leisurely than I would ordinarily have, determined to enjoy the experience. As I continued I fantasized about both Karen and my sister, loving that they were watching me but not knowing about their own starring role in my perverted daydreams. I added a few sound effects as well, grunting, gasping and saying fuck a few times, until I finally came and there was no need to act as I let out a long moan of pleasure and my dick shot its load all over my stomach.
I think I'll stop there and relate the consequences of what I did another time. Hope you enjoyed reading this as much as I did writing it!