I was 17 and dating Lisa, who was 15. There was just over two years difference between she and I. Lisa's younger sister, Barbie, was 13. Maybe 14. Where Lisa was a very chesty girl that carried a couple extra pounds, Barbie was a tiny little thing that hadn't really started to blossom.
I used to spend a lot of time at Lisa's house and those two fought non-stop. One fight, Barbie blurts out "I'm going to tell Mom what you and HIM are doing in your room when she isn't home!!!!"
Lisa countered this with "well, I'm going to tell all your friends about how you spend all day playing with yourself!!!"
Of course Barbie did the whole "I do not, that's sooooo gross" thing.
From there they screamed a little more and then it was dropped. Lisa didn't rat out Barbie, and Barbie didn't rat out Lisa. Not that there was a lot to rat out Lisa for at that time, we barely did much except kiss at this point.
A couple weeks later I went to Lisa's. She was supposed to get out of work at a certain time. Barbie told me that her work was keeping her a couple hours longer. We sat in the living room watching tv and Barbie excused herself and said she was going to go lay down.
After a couple minutes my mind started whirling. I snuck down the hall and tried to listen. I thought I heard Barbie breathing a little heavier, but I couldn't be sure. Her room was on the first floor, I snuck outside and around the house.
Their house was basically in the woods, and the side of the house where Lisa and Barbie's room was located there were no other houses. There window was open (with a screen) to let in the breeze. I got to the window and peeked in.
On the bed, stark naked, was Barbie. She had the pillows propping her up a little, legs spread wide, and a finger furiously rubbing up and down on her clit. Her nipples on her small breasts rock hard, her stomach rising and falling.
Being a 17 year old boy that would get hard at the sight of a girl in a bar in a Sears catalog, I was hard as a rock. I pulled my shorts down around my ankles and started to stroke my cock, eyes locked on Barbie.
She would slow down on her clit from time to time, eyes closed always, and slowly put a finger half way inside her. She'd take the finger out and then start rubbing her clit again.
This was the first time I had seen a completely naked girl like this in real life. It didn't take long and I was cumming, shooting my load on the side of the house and the ground. Not wanting to get caught, I pulled up my shorts and went back inside.
That summer I took every chance I could to be at Lisa's house when she wasn't home. Almost every time I was treated to a Barbie show.
Barbie was a freshman when I was a senior, and Lisa and I broke up later while in college. Ironically Barbie attending the same college and, after not seeing her for years, we ran into each other at a party. We got very drunk and laughed, talked, sharing old times.
It was then that she asked me if I used to watch her. I think the look of horror in my eyes freaked her out, she started laughing. She told me that she knew I was watching, she heard the foot steps outside. She said something like "everyone likes feeling like they turn someone on, and I always had one thing up on my sister...her boyfriend jerking off and looking at me."
I graduated college that spring and never saw her again.