Back when I was 14 or so, a friend and I had been fishing and returned to his house to pack our gear away and clean ourselves up. I had just finished showering and was toweling myself dry when his older sister Judi banged on the door saying she was desperate to pee. I gingerly unlocked the door at which point she burst in and told me to face the other way whilst she peed. I stood with my back towards her holding a towel across my front as she dropped her panties and began to pee. I thought it was kinda gross having to listen to my friend's sister pee, but was more concerned that she could see my butt. I started to wrap the towel around me but she said 'Don't cover up, you have a nice looking butt'. I couldn't believe what I was hearing! I continued standing there not knowing what to do. Then Judi said 'I'll have to change these panties as they're damp' at which point she playfully threw them at me and they landed on my shoulder. I was now staring at a pair of Judi's panties, a pale pink pair which were obviously damp in the crotch area.
To my horror my cock began to stiffen, as Judi approached me from behind. Being 6 inches or so taller than me Judi grabbed her panties and looked over my shoulder at my thickening cock. 'You've a nick looking cock too' she said and reached around from behind, pulled the towel to the floor and grabbed my cock in her hand. I froze on the spot as Judi began stroking my cock, her other hand gently caressing my butt down to my ball sack. 'I bet you'd love me to wank you off' she whispered in my ear, as my cock was now rock hard. 'I've got no panties on and pussy is all wet' she said as she quickened her strokes. I could feel that sensation down between my legs as my knees began to tremble. 'That's it, cum for me' she said, as I released 3 or 4 ropes of cum across the bathroom floor. 'Wow, that's some load you've shot there' said Judi as she milked my cock dry. 'You'd better clean it up' she remarked, smacked my butt and walked out of the bathroom. I dare not tell my friend what had happened, and Judi always gave me a sly wink each time I saw her after that episode.