It started as an innocent enough evening. I went to dinner at my sister's, and we planned to play a game after dinner. My sister had recently remarried, and her new stepdaughter was home that night. We'll call her Casey, for now. I knew she had just turned 18, but didn't attend the party, because I didnt really know her that well.
When I got there, we finished setting the table for dinner, and my new niece came out and sat at the table beside me. She was wearing a cute top and leggings, nothing overly sexual, but looking back now, I realize that was her first tease. She suggested Cards Against Humanity for the game, and she would giggle every time she got dirty cards, and showed me a few of them.
We had a few drinks while we played, so I wasn't feeling up to driving home right away. I set up a spot on the couch, so I could sleep off the alcohol, and went to sleep. When I woke up, Casey was standing over me, with her phone in hand, and mine dinging on the couch beside me.
"You should check that," she said, "and tell me what you think."
I reached for my phone and put on my glasses, only then realizing that casey had changed into a pair of volleyball shorts that were too tight even for her, and a barely there camisole top.I could feel my cock stiffening, looking up and down Casey's body, until my eyes adjusted to realise that my phone was dinging for about 30 different pictures of Casey wearing lingerie, stripping out of it, and masturbating for me.
I looked up at her, totally speechless, and she knew instantly that I couldn't say what i was thinking. She walked over to me, straddled me, and slowly leaned in to kiss me.
"Wait," I said, "we shouldn't do this." I said it, but even as the words were coming out of my lips, I was reaching out to pull her in tighter. I had one hand firmly gripping her ass, while the other was squeezing her breast, and she was grinding against me.
She leaned down to kiss me, and pulled off her top, exposing her perky teen tits for the first time. "I've been waiting for this," she said, "you're mine tonight." Casey reached down, and freed my cock, which was getting stiffer with every passing second. As she slowly lowered herself onto me, she began trembling and moaning.