When I was 15 I left for school and my sis was staying home sick because she had a doctor's appointment at 12 which my mom would take her to. I got two blocks away when I realized I had forgot my homework turned around to go get it. As I approached my house I could here my sister calling our dog, he would often run loose. She stopped calling as I was about a block away. As I got near the house I stopped and talked to my neighbor who was raking his leaves for a minute then headed home. As I walked thru the front door and up the stairs to my room I saw my sisters door cracked open and the weirdest noise coming from inside her room. I walked to the door and looked inside and froze. My sister who was 13 was lying buck naked on her bed with her legs spread and a jar of peanut butter in one hand and our dog licking peanut butter off her between her thighs. She was moaning louder and louder oblivious to me. My cock was hard as a rock and without me even realizing it I pulled it out and was stroking it. I had tried to sneak peeks of my lil sister before when horny but that was it. I now had my cock in my hand yanking back and forth watching my dog lick her pussy out. The next thing I knew I was shooting my load all over the door and I moaned out loud and she heard me and jumped up and started crying. I was so freaked out I ran into my room. I thought about what I had just saw and decided I wanted more. I went to her room, door now locked and asked her to open it. She cracked the door I went in and she begged me not to tell mom or dad. I wouldn't of course I mean how could I but she was crying and telling me she would do anything, all my chores, give me money, anything. I asked if she really meant anything and she said she did. I unzipped my pants and told her to touch my cock which was rock hard again. I then had her jerk on it and made her suck on it. She said it was her first and it was my first. I then started rubbing her pussy and told her I was going to fuck her but she begged me not today that she was ill so I said ok but she had better keep her promise or I would tell mom and dad. This led to me molesting her over and over till I went away for college. I even had our dog going down on her while she blew me. I knew she liked it and so did I and the dog seemed quite happy. This is the first time I've ever told anyone this I mean who could I tell anyway not my GF or buddies, Im glad I found this site.