This happened to me several years ago when I was 14.
During spring break my sister was home from college and brought a friend, Karla, to spend the week.
At the time I had hit a growth spurt but my build had definitely not kept up so I was a skinny, inexperienced fellow. Karla, on the other hand, was a dream (at least my dream). Short and trim and altogether pert. However, I did not have much contact her or with my sister for most of the week.
Towards the end of the week my parents were out of town and I was home alone. Karla and my sister had gone out. I was perpetually horny and about 11:00 I was on the couch zoning out and mildly playing with myself. It was then that Karla and my sister came home unexpectedly early. Actually Karla came in first, followed by my sister. They both said hi and went upstairs to the bedroom. Seeing Karla made me focus more on my masturbating and I started going at it again.
Imagine my surprise when I looked up and saw Karla standing at the doorway to the kitchen. I quickly pulled my hands out of my sweats and put a pillow over my lap. I was very embarrassed. Karla had an expression that I cannot describe except that it seemed to be very curious. She apologized for interrupting and went into the kitchen to get some water.
When she came out she came over to the couch and sat next to me. She said that my sister had passed out, but that she was not sleepy. She wanted to know if she could watch tv with me. That was just fine with me. We flipped channels a little and she suddenly asked if I had kissed a girl yet. I answered yes and started to explain, but before I could finish my sentence she asked if I wanted to kiss her. I was amazed when she suddenly leaned over and kissed me on the lips. She said that she'd seen my playing with myself even before she came in the front door because she'd walked by a window and seen inside. I wasnt able to say much to that, but didn't have to because she kept kissing me. I was starting to groan and I kept trying to look down because she was just wearing a long t-shirt and it was riding up her legs and I could see her panties. She caught me looking and asked if I liked that and then put her hand on my lap to feel my hard cock. I almost came on the spot.
I started to reach for her but she said there was a rule - no touching beneath the clothes. I could touch her boobs through her shirt though - and I did. Then she stood up and asked me to lay down on the floor. I did and she pulled her shirt up showing me her panties and then sat on me, straddling me. She started rocking her hips back and forth and side to side. Then she bent forward and we kissed while she continued rocking. It did not take me long to cum, thrusting myself against her (and masking a mess of my sweats). It was uncontrollable for me and as I quivered I held on to her very tight. When my convulsions stopped she rolled off me and we kissed a little more.
As she lay there she started rubbing her panties and took my hand and put them between her legs so that I started rubbing her panties too. She told me again not to go beneath her clothes, but I could rub all I wanted - and I wanted a lot! As I rubbed her panties she started pulling on her boobs and began to have her own convulsions as she came. Just as she was coming she reached over and grabbed my cock which had gotten hard again and squeezed it. I almost came again right on the spot.
When she caught her breath she said that she had to go get cleaned up, and obviously I did too. She kissed me again and said 'you are as good as your sister'. I wasnt exactly sure what she meant by that and was too awestruck to ask. I wish I had and maybe have been bold enough to follow her back into the bedroom.
I did not see her after she left that week and never asked my sister about her. Maybe I should. :-)