This happened when I was fifteen and my sister was seventeen. We have always been cool with each other and get along well.
Late one night I felt the urge to jack-off and I thought everyone was in bed. I was in my room and decided to watch some porn clips on my laptop. I also used some headphones so I could listen to them loud with no one else being being able to hear them. I had the laptop on my bed and began stroking as I lay there. All I was wearing were some boxer-briefs. I had my cock out and was pounding away. I went at it for about ten minutes. The feeling was great and I was about to shoot. 'Oh yeah, oh yeah!' I shot my load on my stomach as I moaned with desire.
I then took off my headphones and heard my sister say, 'That must have felt good.' She had a strange look on her face. I was shocked. She must have knocked and I didnt hear her come in. I can't believe I forgot to lock the door. I didn't know what to do. The covers and my clothes were on the floor. I quickly threw my cock back in my underwear. I had jizz all over me, I didn't know what to do. So I asked her to get me a towel. She walked over to my desk and gave me the towel that was there. She then walked out and closed the door.
I cleaned up and got dressed. I went to the living room where my sis was watching tv. I told her I was so embarrassed and asked her not to tell our parents I had porn on my laptop. She said she wouldn't. We laughed a little and I told her thanks and went back to my room. Ever since then my sister has caught me six times. I have caught her twice though. We still laugh about it now as we did then.