My parents went to a dinner party tonight, so I was left to look after my sister, 13. There was a big thunderstorm, and the lights went off, so I said, we can't watch tv, might as well go to bed. I had just got into bed when sis came in the room. She said she was scared of the dark (normally there was a landing light on), and she climbed in beside me. It was cold, and I could feel she was cold, so I let her snuggle up beside me. I had never thought of it before, but now I realised her body felt nice, close to mine. She was wearing a nylon nightie, and it had ridden up when she climbed in the bed. Now I could feel her bare bottom. I was wearing just my underpants, and realised they were becoming uncomfortable as my cock became hard. I eased them off as unobtrusively as I could... My cock immediately sprung upright! It was rubbing against sis' pelvis, and it felt soooo good! The hand that wasn't wrapped around her bum now found her 'front bottom', and gently caressed it... I sensed her tense and her breathing quicken as I slid a finger between her lips.... She was hot and slippery inside... I felt her moving against my finger... She was using me to masturbate herself! At this thought, my cock grew harder than ever before, and I gently manoeuvred her hips over me so her pussy rested on my cock. now she was rubbing herself hard against me.... Her hand grasped me and I felt her pressing me against her vagina... It was too tight, then suddenly I was inside her... She gave a little whimper, then slid herself down so I was all the way in. Then she sqirmed around and I could feel her fingers rubbing her clit.... I grasped her buttocks and began to thrust slowly ... I felt her vagina grip me with every movement... Pretty soon I knew I was going to cum, so I pulled out and turned her on her back... Her little hand was still furiously fingering her clit as I knelt over her and shot my sperm all over her...