When I came home yesterday, I dropped my bag and sandals at the door and went upstairs. I heard some music playing, and when I got to my sisters room, I saw her and her boyfriend having sex. I usually go to a friends house after work, so I guess they thought they had more time. I couldn't stop watching him going in and out of her and her feet bouncing in the air. Watching them got me really horny, so I pushed my pants and undies down to my ankles and started rubbing my damp pussy. They were facing the other way so there's no way they could have seen me, so I just stood there masturbating in the hallway.
I was there for about 10 minutes and I heard her say she was going to cum, and when she did her legs stiffened and her toes spread apart. He didn't stop though, and I stayed focused on his penis sliding in and out of her. I didn't want to miss any of it, so when I got close I stopped and started again. She came a second time and almost begged him to stop, but he told her not yet and almost started going faster. She started whimpering and pulled him tighter, but he kept going. Pretty soon he said he was going to cum, so I started rubbing as fast as I could. He then pulled out, yanked off his condom and came all over her belly. This did it and I came like never before, and I had to bite my tongue to stay quiet.
He then laid back down on her and they started making out, and I quietly pulled up my pants and went back downstairs and acted like I had been watching TV. When they came downstairs, they had no clue I watched them.