During the summers our parents would let us stay home alone because my older sister was there and was responsible enough to watch me and the house. One summer when I was about 13 and she was 16, a lawn mower woke me up. It was still early so I tried to go back to sleep but it was no use. I threw on a pair of shorts and went downstairs. When I got to the living room, I saw my sister standing at the window, watching the neighbor boy mow the grass. Since the windows were open it was kind of loud so she didn't hear me walk in. I was about to say something, but I saw that her panties were down around her bare feet. I snuck back around the corner and spied on her, and I saw that she was rubbing herself.
By that age, I knew exactly what she was doing, and seeing her do it was getting me horny too. I hid there behind the doorway but I still had a good view of her. I stayed there for a few minutes before I saw her kick off her underwear and sit down, spreading her legs and giving me a great view of her bare body. Seeing her shove her finger in and out of her wet pussy was more than I could handle, so I took off my shorts and underwear and started jacking off too. She must have been at it for a while, because she came pretty fast. When she did, her feet pointed and her toes spread apart, and she what said I'll never forget. She said 'Oh fuck! Fuck me with your big cock! Cum inside me!' By this time I was dripping so I used my underwear to wipe off with. I guess she was still horny, because then she took off her robe, lifted her legs up and started fingering herself even harder. Seeing all of this was more than I could take, and shot a huge load into my underwear. I guess I banged against the wall or something and startled her, because right when I came, she looked over and saw me standing there naked. She then yelled out 'quit watching me masturbate you fucking perv!' She then got up and I grabbed my shorts and ran back to my room. Right after I closed my door, she barged in with both of us still naked. She pushed me on my bed and held me down, yelling at me and making threats. When she had to catch her breath, I told her if she left me alone I wouldn't tell mom and dad what I saw, who were kind of strict about this stuff. She thought about it for a few seconds and said fine, but I had do stay in my room until she was done. I said ok and she started to leave. Before she walked out she turned back around and said that if I watched her masturbate again she would kick my ass, then slammed my door.
My heart was pounding because I was scared, but thinking about my sister on top of me naked, I was incredibly horny again. I started stroking and a couple minutes later I blew another load into my underwear again. I put my shorts back on and laid on my bed for about another 20 minutes before she walked back in, this time with her robe on and tied shut. She said she was done I could come out now. The rest of the day we didn't say one word about it and I never told mom and dad what I saw. For a while after that though, every time I masturbated I thought about what happened.