My 18-year-old sister has a really hot figure; 34c breasts, tight waist, wide hips, a lovely wide arse; the way I like it with my girlfriends. One day, I climbed upstairs to her room to go see if she wanted to hit the beach with me. I saw her in a strapless white dress tucked up past her buttocks. She was facing away from me rubbing her pussy from the front through her panties.
Instantly, I got hard. She started to moan softly, jiggling her arse around. She didn't notice me. I walked up to her, held her hips, and unzipped my pants letting them fall to my feet. I took out my hard five-inch penis and stuck it between her legs on the outside of her pussy. I humped her back and forth; she gasped loudly, reached behind her and found me. She pulled me to her, bucking her upper thighs along my shaft. Soon she came, and her orgasm made me cum. Her panties were soaked both sides.
We giggled afterwards, then she chucked me some Kleenex and left for a shower. We've not done anything since; this was on Tuesday.