One night I woke up around midnight to go to the bathroom, but when I was passing my older sister's room, I could hear some noises. I pressed my ear against the door, and I could hear some buzzing, heavy breathing and her moving around on her bed. I have heard her and her boyfriend having sex before, but hearing her masturbating got me wet in an instant. I stood there listening for a few seconds, hearing her moaning and muttering the boyfriend's name, but I couldn't take it anymore. I hurried back to my bedroom, tore off my jammies and jumped on my bed with my hand stuck to my soaked vagina.
I started rolling around on the bed, with the cold sheets touching every inch of my body while I stroked and fingered my wet pussy. This felt really good, but it wasn't enough. Making sure everyone's door was shut, I walked down to her room, still naked, and stood outside her door and kept fingering myself while I listened to her. I knew she was about to cum, because she started grunting and bouncing on her bed. I started going faster, and right after I heard her cum, I bit my lip and stuck my finger deep inside, brushing my G spot and exploding with a huge orgasm. I didn't want to get caught so I tiptoed back to my room and closed the door with my finger still inside me. That night I actually fell asleep naked, thinking about what I did.
When I got up the next morning, after my shower I went downstairs in nothing but my bathrobe and acted like nothing had happened when we all sat down for breakfast. It got me horny again wondering if my parents knew that both of their daughters masturbated last night. After I ate, I went back to my room, opened up my robe and came again.