All characters are over 18.
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My parents adopted me when I was 12. They cared for me in a way a family would care for their biological son. I finished school and I'm going to varsity next year.
They adopted my sister when I was 13; she was 11 at the time. We grew up together but it never felt like she's my sister, maybe it's because we were both adopted.
Years later I came home early one day from my part time job at my father's company. My sister Becky was at home spending time with one of her best friends. I wanted to scare them so I opened the front door quietly and tiptoed to her room. When I got to her room I paused and listened. The door was closed but I could hear clearly as they were talking a bit too loud to one another.
"Take your shirt off you slut!" my sister's friend Ricky said angrily. "And your bra!"
I was shocked but continued listening; I wasn't sure what was going on but I wanted to find out.
""My God your tits feel great, uh I'm getting so wet." She said again after a short pause.
There was silence for a while then she told my sister to suck her nipples. I could hear soft groans coming from the room and made the conclusion that my sister was sucking her friend's nipples.
I got an erection and wanted to jerk off but stayed at the door. The next line from the friend shocked me.
"Want to lick my wet pussy you slut?" came from the friend.
"Give me your sweet pussy, I'll lick you all you want." My sister said in a desperate voice.
I heard the bed squeaking once or twice then it was quiet. Her friend started to groan loudly so I thought that my sister was licking her pussy. Her friend shouted "Oh God I'm going to cum" followed by a loud groan. I turned around and headed for the bathroom.
My cum shot out onto the wall as I reached my orgasm. I cleaned the place up then went back to my room.
I went to the kitchen to get something to eat when I walked into them.
"Hi... when did you get home?" my sister asked with a nervous look on her face.
"Hi, about 5 minutes ago." I said. I knew they would think I didn't know a thing. I greeted her friend and walked into the kitchen blushing from the mental image of them naked and licking each other "downstairs."
They talked and joked as if nothing happened between them. I turned around and headed for my room when my sister spoke again.
"We're going for a swim, want to join us?"
"Uhm, I don't know... I uh... no it's okay." I finally got out and went to my room.
I locked my door, put my sandwiches down on the bed and pulled my pants down again. I got another erection when I thought of them all wet in the pool. This time my cum shot out on to my carpet and I had to clean it up. I ate my sandwiches while watching an episode of Clone High on my computer.
I went to my window and looked out onto the pool. They were splashing each other and just having fun. I never thought I'd see the day that I knew a sexual secret about anyone, much less about my sister.