I left her naked and alone at the computer. I grabbed a cold beer from the fridge and sat down in my easy chair.A couple of hours went by and she was ...till intensely reading at the computer. I had to do a double take as I looked at her and saw her legs slightly spread. I could tell her shoulder muscles were moving, but I was in the wrong spot to see anything. I moved without her noticing and saw her fingers rubbing her clit. She must have been doing this for a while as I saw cum dripping off. They both smiled and drank their drinks.For the remaining duration of the trip, they held hands with Greg driving, lost in his thoughts about Sarah, and Sarah humming some tune in contentment.Relatively soon, they arrived at his parent's house. He parked in front, Sarah's side of the car closer to the house. He got out of the car, did a quick check of himself. He noticed a spot of something near his crotch and decided that an untucked shirt would hide it. He jogged to the other side of the car. We sat down on the bed we started talking. You know you turned a lot of heads at the party the other night, there were a few guys interested in seeing you. Would you be interested? Between being tipsy and so horny I probably would have agreed to almost anything at that moment.I said I would not know what to do? I am not into men, I am not sure. She reaches over to my panties and started to rub my cock, your mouth says no but the rest of you say yes.I don't know, I don't even know what to do or. She knew that it could only be Brianna. She looked through the crack in her friend's door and gasped quietly. Alan was pounding into his little sister's pussy. She smirked. “So that's why she didn't care about Alan and I,” she said to herself. She rushed to the bathroom and sat on the edge of the tub. She slid a finger into her twat and began fucking herself. She bit her lips, imagining A threesome with Alan and his sister. She came again and again. It was so sexy to think about. Alan groaned.