Horny and bored, I sneak into my girlfriend's room while she's out, only to find her younger sister, a tantalizing teen with a questionable sense of modesty, changing. I can't help but stare at her perky tits and round ass as she bends over, completely oblivious to my presence. My cock twitches, and I quickly pull it out, stroking it as I watch her. She finally notices me, but instead of being embarrassed, she seems intrigued. She watches me jerk off, biting her lip, and before I know it, she's joining in, rubbing her pussy through her panties. I can't hold back, and soon, I'm cumming all over her floor, leaving a sticky mess for her to clean up.