I was going out one Friday night and my younger sister and her friend had bought a bottle of vodka and some films to watch for a night in which they did every now and then. When I came home that night after midnight the front room tv was still on, the large vodka bottle was empty and my sister was blind drunk and falling about laughing and her friend was comatosed flat-out on the sofa in her nightdress. My sister said she was getting cold and was going to bed and would I help get her friend off the sofa and up the stairs with her otherwise she would freeze to death in the middle of the night if she was left there. We pulled her up on her feet and I threw her over my shoulder and followed my staggering drunk sister up the stairs to her room. When she got to her room my sister said just to throw her friend in the spare room just in case she was sick in the night. My sister closed her door and switched her light off and I heard her banging around and then crashing on her bed to go asleep. I made my way to the spare room and pulled the duvet back on the spare bed and put her friend down who was still sleeping in a drunken stupor. As I let go of her and moved back I couldn't help but notice that her nightdress had rode up her back slightly, exposing her black silky knickers. Her bum was protruding slightly off the bed and she was facing away from me towards the wall snoring. I suddenly was overwhelmed by the urge to cum. I very quietly got down on my knees next to the bed and I carefully peeled her knickers down trying not to rouse her,  just far enough to see her round, smooth arse and heavenly hairy pussy jutting out from behind. I felt like fucking her but I knew I couldn't.  I quietly undid my jeans and slipped them and my shorts down to my thighs and my cock sprang out. I was so nervous that i could feel my heartbeat in my ears. I started quietly jacking myself off at the magnificent sight that was in front of me and it was so fast, maybe 15 seconds and I came hard knowing that I was just inches away from her pussy masturbating. I lifted the bed sheet off the floor and shot my huge load under the bed, wiping the remainder off my cock with the sheet. I quickly fastened my jeans up and stood up, pushed her in the bed and put the duvet over her and flicked the light out as I left the room. She never noticed a thing.