This happened a few years ago; I think I was 16 at the time.
School was really boring that day; I just couldn't pay attention to anything. In my last class of the day, I got really horny. It was a Friday, so if I didn't make plans, I would have all weekend to jack off. I kept looking up at the clock waiting for the bell to ring. Time was moving so slowly; It took forever. It finally rang, and I ran out of class and hopped on the bus. (My car was in the shop because my friend had a minor accident in it.) I couldn't wait to get home.
I got home, and to my luck, nobody was home. There was a note saying that my parents were at the grocery store. I was just going to get into my room and go at it, but now I had the whole house for me to 'play' in.
I made sure the doors were locked and the windows were closed. I got naked and sat on the couch, looking at a porn magazine of which I had recently gotten hold. I got a boner, and started pumping my hard cock. I squirted some lube on my hand and started really going at it. I opened my eyes to look at another picture, and there was my little sister! I had completely forgotten she was sick at home that day, and I became worried.
'So, this is what you do when nobody is around, huh Danny?' she said.
My sister was 13, had brown hair, and was really short. She had a decent chest and a nice ass, but I never thought anything sexual about her. She smiled at me and said, 'Don't stop, I want to see!'
I nervously started stroking my cock again, but I had gotten soft. 'Here, let me help,' she said to me. She took off her shirt, and started shaking her tits in my face! I instantly got my boner back, and started pumping away at my cock again. 'This won't do!' she said, as she pushed my hand away and started doing it for me. I came almost instantly. I jizzed all over her hand, and some even got on her naked tits. It felt so good! She got some tissues and cleaned me up, and went back to her room.
Later that day, my mom walked into my room. 'So, I hear you were masturbating today while I was gone,' she said. I was horrified, I couldn't believe my sister told my mom. 'It's okay, everyone does it. I do it all the time when dad's out of the house. So does your sister. You have nothing of which to be ashamed.' I was surprised she didn't get mad. She was normally against anything sexual at all before marriage. 'Want some pointers?'
I was in awe. I didn't know what to say, or what she would do. I slowly nodded my head and she walked over to my bed. 'Drop your pants, Honey.' I did as I was told. She looked at me and started fondling my balls. 'Wow, you're bigger than Dad. How often do you jack off?' I told her about once a day, as she gently worked her way up from my balls to my cock. She spit on it to get it lubed up, and start pumping as fast as she could. It was great, and I jizzed within five minutes. I got it on her face, her hair, and on her blouse. I think it was the biggest load I've ever shot, and it was all over my own mother!
'Good job!' she said. She grabbed a towel off my floor and cleaned us up. She walked away from me and headed torward the door, turned and said, 'Maybe all three of us should do it together sometime!'
She laughed and left my room. It was one of my best masturbation experiences ever.