One night, I was woken up late when I thought I heard someone talking. I stayed silent to try to hear who it was and figured out it was coming from my sister's room. I scooted closer to the wall to hear better and I heard her boyfriend too. I thought they were just chatting, but then I heard my sister let out a little moan. Her mattress started making a little noise a few seconds later, and I realized they had started having sex. The thought of them being naked together shocked me a little at first, but the more I heard her moaning, the hornier I got. He must have been really good at it, because her moans turned into quiet grunts. When this started, I had to masturbate, so I pulled off my pajama pants and undies and started rubbing myself. I closed my eyes, first thinking about them together, and then thinking about him being inside me, taking my virginity. She then let out a long and low 'ohhhh' sound and quietly started saying 'I'm cumming! I'm cumming!' which made me cum as well, making me wiggle and twist around on my sheets. He didn't stop until he came though, and afterwards the room went silent.
I didn't hardly sleep at all, my mind kept racing and my pussy kept getting wet over and over again when I thought about what I heard. I guess I dozed off eventually because when I woke up it was daylight out. Not only that, but I could hear them doing it again. Since my parents both had to work, they didn't have to worry about them, so they were really going at it. This time she was flat out yelling and moaning, saying 'fuck me' over and over again. I had to masturbate again, so this time I got completely naked and sat against the wall, listening as closely as I could. This time I came twice before they were done, draining all my energy and making me fall back asleep. I woke up about an hour later lying on my belly, still naked on top of the sheets with my legs wide open.
I took a shower and went down to have some breakfast, and before I finished they both walked downstairs, kissed and she let him out. She didn't say anything about it, but she had to know I heard them. They were together for another couple of months and I heard them a few more times, and every time I could, I masturbated along with them.