Big Booty Sister Mooned Me

I was 12 and my Step Sister was 15ish. I had never seen her naked before. Hell, ive never seen any girl naked at that point. I might of seen her in her panties one time in the year we had lived together for. I'll be honest, she is absolutely gorgeous! Big athletic ass, strong legs, etc. I would randomly moon her sometimes if we were alone in the house together. She would laugh and roll her eyes, laugh and say some shit like "SERIOUSLY?! You're going to make me throw up lol!" One night we were alone, ill never forget it, I was laying on the floor watching TV I think and I seen my step sis out of the corner of my eye. I thought she was going to sit in the chair next to where I was laying. Instead she stands almost directly over me. Each of her feet were about where my knees were. She pulls down her shorts and panties, that's if she was wearing any, I couldn't really tell. She pulls her shorts down near her knees, bends over enough that I could clearly see her pussy lips and even a bit of her butthole for probably 2ish, maybe 3 seconds. Then pulls the shorts back up, laughs, and said "PAYBACK!" Then she left the room... I was sort of in shock. She had never done anything like that before. I remember getting so tuned on by her mooning me that I kind of got obsessed with my step sisters ass and puss.... I had never really been turned on sexualy by her prior to that event. But afterwards I did some risky stuff to be able to see her bare ass and puss again. I think most of boys my age would of done the same. Yet, is was some Very risky stuff that now I feel pretty bad about.