When I was younger, during the summer I was always home with my mom and my sister who is one year older than I am. This was before wireless internet really took off so we just had the one computer to get online with, and like most people then it was in our 'computer room'.
This morning I had to get up early to cut the grass, and since it was the middle of summer it was already hot, even in the morning. When I was done I needed a shower, and after I got out I just put on a pair of basketball shorts since I wasn't going anywhere else. After I had breakfast I was watching TV and my mom told us she was running to the grocery store and to behave ourselves. I figured that would be a good time to get on the computer and to play with myself. As soon as I logged on (remember dial-up?) I started looking at naked pictures and my erection started to grow. I listened to make sure I was alone and pulled the waistband down, exposing my cock and started stroking.
Sometimes I think I have the worst timing, because right in the middle of it I heard my sister coming up the stairs so I put it away quickly and tried to hide my tent and switch web pages as she walked by. Her bathroom door shut and locked so I went back to stroking myself. Again, it was really starting to feel good when I heard the flush and the door unlock, so I covered myself back up for the second time. Needless to say, I was starting to get frustrated. After she went by I pulled out my cock for the third time, but this time I was determined to cum so I kicked off my shorts leaving me completely naked in the chair. Once again I heard her on the stairs but this time I thought 'screw it' and just kept on going. When she passed by the room I could see her slow down out of the corner of my eye, but I acted like I didn't see her, or that I didn't care. She said out loud 'oh jeez...!' and hurried back downstairs, but that just made it better. Knowing my sister caught me made me blow, shooting a huge load of cum all over my chest.
I shut it down and went back to my room to clean up, then put my shorts back on and went downstairs. When my sister saw me she asked with an attitude 'oh, you're finished?' All I said back to her was 'yup' and turned the TV back on. After a while I did start to feel a little ashamed about it, but I figured what was done was done. That was the only time I ever went naked in front of her, and when I needed to masturbate I made sure I had enough time to finish without getting caught.