We live in a small culdesac and have big wooden fences around our back yard. I occassionally will sunbathe in the back yard. One day in July my parents were going out of town for the day. They also let me know that my younger brother, who is thirteen, was going to be at the beach with his friend and their parents. Figuring that I had the whole house and yard to myself I decided to sunbathe nude in the backyard. I don't have to worry about neighbors or anything. So I got out there and took off my bikini then covered myself with suntan lotion and laid out. At one point I took a look around and thought I saw the curtain in the bathroom move. I wasn't sure but decided to keep an eye on it and for sure it was. Someone was in there an looking at me. It didn't bother me much, but I was a little curious about who was in the house. So I started to get up and make my way to the house. I didn't see the bathroom curtain move again, so I figured whoever was doing it stopped. I snuck in the back door and made my way down the hall and to the bathroom door. I peeked around the corner and there was my younger brother at the window. I could tell he was doing something but not exactly sure what it was at first. Then I realized he was masturbating while looking at me. At first I was a little freaked out, but then I thought I would mess with him a little. I yelled "hey, what do ya think your doing?" He jumped and jumped hard. I could see him trying to tuck his penis back in his shorts and play it off like he wasn't doing anything, but I could tell. I sat there peeking around the corner and he sat there breathing real hard and just looking at me. I just sat there still peeking around the corner. He told me he wasn't doing anything. I said oh yeah, then what were you looking at through the window? He again told me nothing. I asked if he was sure and he noded. So I then brought the rest of myself from around the corner and showed him my still nude body. Quickly I saw his eyes jump all around from my breasts to my vagina. I also noticed that he was getting real hard again. I then asked him if he likes what he sees and he just nodded with his mouth open. I then asked him what he was going to do about it. He kind of wiggled his head in a I don't know fashion and continued to stare at me. I decided to walk toward him slowly and when I got him I ran my hand along his shoulder to his neck and tickled him a little. I could see his eyes just roll in the back of his head. I then reached down and grabbed his hand and placed it on my breast. With his fingers in my I started carressing my breast and nipple. I also leaned my head down and started to kiss him. I then started pulling him out of the bathroom toward our parents bedroom across the hall. He just followed me and when we got into the room I pushed him on my parents bed. I then crawled over him and strattled him. I could feel his fully erect penis on my vagina through his shorts. I then leaned over and put one of my my nipples in his mouth and he began to tease and suck on it. I also started grinding against his penis really hard and I could tell that he was really anticipating what I was going to do next. I asked him, "do you like this?" He said, "YES!" I then asked him if he wanted me and he replied, "YES!" So I then really started grinding against him and I could tell that it was feeling really good. I kept going until I could see that he was getting ready to climax and just before he did, I hit him in the face. I looked at him and told him that if I ever caught him looking at me again I would do more than just hit him. I then climbed off of him and went in my room to get dressed. But before I did I had to take care of myself. I didn't realize that my brother was so big and feeling his erect penis against my vagina through his shorts really turned me on. If I catch him doing that again I may not be able to stop myself. I hope I do!