One day, I went with a friend of mine over to his house for some relaxing. We were in the basement watching TV when we heard his older sister come home from work. Since she didn't come down and see what was going on, I guess she didn't know we were there.
After a while, we heard water running, so we figured she was taking a shower. After it stopped, I had to go to the bathroom, so I went upstairs and heard some loud music. I wanted to see what was going on, so I went upstairs. I pulled off my flip flops to stay quiet and tiptoed down to her room and peeked inside. Since she didn't know we were there, she had left her door wide open. When I looked inside, I saw her wet towel on her bed and she was sitting at her computer completely naked. I couldn't see between her legs, but I did see that she had large boobs. I must have stood there for five minutes watching her type when her phone rang. I could tell by how she was talking that it was her boyfriend, and I heard her tell him that she was sitting there naked.
By this time I was completely hard, so I pushed down my gym shorts and started masturbating while I watched her. She talked and giggled for a little while before she opened up her legs and started rubbing herself. I couldn't believe it, my best friend's sister was masturbating right in front of me and she didn't even know it. After about 10 minutes, she started getting getting wild, moaning and telling him how good it felt. By this time, I was right on the edge. I knew she was too, because her head was thrown back and she was getting louder. I tried to hold out longer, but I couldn't. I felt I was about to cum, but I realized I didn't have a tissue or anything to blow in. At the last second, I held one of my sandals to my penis and blew a huge, runny load on it.
After I came, I hurried to the downstairs bathroom and cleaned it off. When I got back to the basement, he asked me what took so long, so I told him It must have been something I ate. We went back to watching TV for the rest of the day. I don't think either of them ever found out what happened.