My sister and I share the same birthday. We're not twins though. We just happened to be born on the same day, but one year apart. I guess Dad was a busy guy back then. Shortly after I turned sixteen and Marilyn turned fifteen I began to notice that my little sister wasn't so little anymore. At fifteen years old she had already developed a rather large bosom and a very nice, curvaceous body. She was around five feet eight or nine and I guessed her to be somewhere near one hundred twenty pounds. She kept her hair short and parted on the side. She didn't wear a lot of makeup, but then again, she was only fifteen and mom and dad didn't approve of that sort of thing. She had definitely grown into a nice looking young woman. For me, we'd gone from being just brother and sister to her becoming the fantasy of my late night jerk off sessions. As much as I fantasized about her coming to my room for raw sex adventures I knew this was something that was limited to the realm of my oversexed imagination. Still, I enjoyed it even if only in my mind. The first time "it" happened was purely by accident. Mom and Dad had gone out to a dinner party and Marilyn was over at her friends house, or so I thought. I was in my room, on my bed with my computer monitor facing me, watching a really good porn clip. I had taken all of my clothes off so as to really enjoy my time alone. I had poured some oil over my stone hard cock and was slowly rubbing it up and down. Obviously, I was lost in the porn action because I didn't hear Marilyn come home. The way my room is set up the mirror on my dresser gives a perfect reflection of my bedroom doorway. So, here I was stroking my cock in complete abandonment when I noticed a movement behind me. Marilyn was standing there. My initial reaction was to pull the bedspread over myself, but something stopped me. Maybe it was shock or embarrassment or maybe something else, but instead of panicking I continued to rub my prick while my sister simply stood there watching. The excitement of Marilyn watching me jerk off was a turn on I had never thought about before. In all my fantasies I had never given a moments thought to her watching me. Before I realized how far along I was I shot a huge stream of cum into the air. It landed on my chest and stomach. A second and third stream hit me in the face and chest. I had never cum so hard in my life. As the last remnants of my orgasm dribbled into my hand I looked up to see that my sister was gone. She must have sneaked out because a few minutes later I heard the front door open loudly and Marilyn came in the house a second time. I went into the bathroom and took a quick shower, dressed and went downstairs to see what was going to happen. My sister was sitting on the sofa watching something on TV. "Thought you were over at your friends house," I said, not looking directly at her. She continued to stare at the TV. "I was, but her mom came home and started in about her not cleaning her room so I left. What are you up to?" So, we weren't going to acknowledge what had just happened. "I was just hanging around watching some Youtube vids." And that was the end of it until a week later. Our folks had gone out again, but this time Marilyn stayed home. I had a plan to see just how far this was going to go. After all, I had thought of nothing else all week. My sister and I were watching TV when I said, "I'm going up to my room to watch some clips". She merely shrugged as if to say, so what. We'd soon see if her shrug was for real. I found a good long video clip to watch and as before I stripped down naked and laid on my bed. I was so excited I was afraid I'd blow my load before Marilyn had a chance to follow me upstairs. I didn't have to worry though because as soon as I oiled my cock and began to jerk it she was in the doorway. This time I said, "Come on in." At first it looked like she was going to run off. I thought for sure she knew I had seen her the last time, but I guess I was wrong. After a short hesitation she walked slowly into my room. I had to focus all of my attention on not cumming right then and there. Marilyn stood directly over me. "You're discussing," she said never taking her eyes off my cock. I continued to jerk off without answering her. "Is it going to shoot soon?" she asked, her voice a little ragged. "Yeah, pretty soon," I managed to say. "Does it feel good?" "Yeah, it feels real good." "Does it make you horny doing it in front of me?" I knodded. Finally she admitted to seeing me cum the week before by saying, "Is it going to be as much as last time?" Before I could answer I started cumming. I shot a huge stream into the air that went over my head and hit the headboard of my bed. A second shot went straight up and landed on my stomach. Several more hit me in the chest and legs. When I eventually stopped shooting I was drenched in my own cum. Without a word Marilyn walked out of the room, but returned a few moments later with a washcloth. "We'd better get you cleaned up before Mom and Dad get home," she said and began wiping the cum off me. It took several trips to the bathroom for her to get me cleaned and I was grateful because I had cum so hard I didn't have the energy to clean myself. When she was done, she said in a sweet voice, "Tell me next time so I can watch from the beginning." For the next three weeks we never got the house to ourselves. But I think this actually gave Marilyn time to get into it fully. It started with little innuendoes, eye contact and deliberate brushing against me. Finally after an eternity Mom and Dad went out one Friday evening. I was in my room laying on my bed fully dressed when my sister came in. "What's going on?" she asked, while avoiding looking at me. "Not much. How about you?" I pretended to read a book. "Same here," she said. We got quiet for a while then Marilyn spoke. "So, you gonna do it tonight?" I put the book down. "I don't know. I thought maybe you'd do it for me..." "What do you mean? You want me to jerk you off?" "Sure, why not"? She shook her head. "No way, Jose! Forget it." "Okay, see ya later", I said and went back to my book. Marilyn looked surprised. "I thought you said you were gonna do it?" I put the book down for the second time. "Oh, I am. But you're not going to be there." "Fine," she said and stormed out of the room. Shit. Maybe I pushed it too far. I waited for a few minutes and was just about to call her back when she walked into my room. "I've never done anything like this before," she said, as if I had thought otherwise. I tried to act matter-of-factly, but my hands were shaking. When I slid my pants and underwear off and sat on my bed my cock sprung out. The head was red and swollen, the shaft equally so. Marilyn gasped. I could't blame her. It looked hard and mean. "What should I do," she asked? "Well, you can pour some of this on it," I said handing her a bottle of oil. Marilyn did as she was told. "Now put your hand around it like the grip on a handlebar and go up and down. " For a moment she paused, then took my cock in her velvet hand and began to stroke it up and down. This was the first time a hand, other than my own, had been on my cock and it felt unbelievable. As Marilyn stroked me up and down I arched my hips in time to her movements. Her oiled hand felt like what I imagined a pussy would feel like and I fucked it as hard as I could. Precum began to ooze from the tip of my engorged cock. Marilyn took my balls in one hand and continued to jerk me off with the other. I wasn't going to last much longer and I told her so. She laughed. "Do you want me to stop," she said dropping my cock and balls? "Oh hell no," I whimpered. She took my cock back into her hands and started jerking it again. "I want to see it shoot a lot," she said. To my surprise my sister let her right hand jerk me as it had, but ran the palm of her left hand over the swollen cock head. This was it. Before I could say anything a huge stream of cum shot out against her hand spraying us both with equal amounts. I moaned and Marilyn laughed. With her left hand covered in cum she placed it back on my cock along with her right hand and resumed jerking me off with both. I pushed into her hands as if my life depended on it. She, in turn, squeezed her hands tightly around my cock as hot cum flew everywhere. I must have shot ten streams before I finally finished and collapsed onto the bed.