This experience happened when I was around thirteen, just entering puberty. I had an older sister, Pam, who could be quite irritable at times. She was seventeen, somewhat overweight and going through a rough patch in high school. I tended to avoid her as much as possible.
But then puberty began kicking in and I found myself thinking of girls often. My sexual curiosity also grew. I tried to find pictures of naked women to look at, but other than some art books in our house, there was little to satisfy my growing curiosity (remember, this was well before the Internet!).
Slowly it dawned on me to try and catch a glimpse of Pam while she was undressing. Hopefully seeing her naked would satisfy my insatiable curiosity. Her bedroom had two doors. The back door was kept locked but led to a second back hallway that was little used. I began sneaking into the hallway and peaking through her keyhole. For several days I set up my little post outside the back door and peeked through the keyhole whenever she was there. But I didn't see her naked. I watched her do everything but take off her clothes.
Finally, it occured to me to take a peek right after she had showered. After several misses due to interruptions, I finally timed it right and got to the keyhole right after Pam entered her room wearing her bathrobe. I remember trembling with excitement and feeling my penis quickly grow erect in anticipation. Sure enough, the robe came off, but she sat on her bed facing away from the keyhole! All I could see was her backside. I waited like what seemed an eternity and then she finally stood and turned towards where I was peeking.
I was amazed at how sexy my nasty, older sister looked! Her skin had a nice pink glow, probably from the hot shower. She was all curves and smooth skin. Even her tummy had this delicious softness to it, which turned me on. But my eyes mostly looked at her well-developed breasts with their bright pink nipples and I stared hungrily at her triangle of dark curly pubic hair.
I was in heaven. My loins were on fire and I was visibly shaking as I kept my eyes glued to the keyhole. This was everything I was expecting and I was terribly excited by what I had just seen, and then I got a treat, which arouses me to this day every time I remember it.
Pam lay on her bed for several minutes, reading a book. I soon noticed that while she read the book with one hand, her other hand was stroking her skin and then moved down and started to touch her pussy. Now I was really on fire, even though I wasn't entirely sure what she was doing.
For several minutes, maybe less, Pam rubbed herself though I had trouble seeing exactly what she was doing, since she was partly turned away. Suddenly she sat up, put the book down and grabbed a large stuffed animal that lay on her bed. She turned and, facing in my direction (thank god!), straddled the stuffed teddy bear. Then, kneeling forward, she began to slowly grind her hips and pelvis on the stuffed animal. I remember watching her luscious breasts sway rhymically to her grinding and humping. Pretty soon, she picked up the pace and humped faster. I remember her face turning red and her long hair swinging wildly, just like her breasts. Then she began grunting. The noise was unlike anything I had ever heard her make. Without realising it, I was rubbing my hard penis, which was still inside my pants, with my hand. I began to feel this hot sensation grow inside of me and then felt for the first time that wonderful pulsating action of my penis as it ejaculated. I looked down and saw a warm dark stain appear in my pants.
I looked back up, peeked through the hole and saw my sister give the teddy bear a few final thrusts with her hips and then watch as she rolled over and collapsed on her bed. I was drenched in sweat, sticky with cum and aware of something incredibly new.
What a memory!