Every day, I spent time with my best friend, a classmate at university, complaining about not having girlfriends. One night, my younger sister Ruru suddenly entered my room when she was supposed to be asleep. I tried to shoo her away, but my friend snapped, "Guys who aren't kind to their sisters don't deserve girlfriends," and urged me to talk with her. Hesitant but persuaded, I kept drinking until my consciousness began to blur. When I woke up, my friend was deep in sleep, and Ruru was tidying up the drinks. Startled by my sudden awakening, she mischievously smiled and abruptly kissed me. She began craving my body. The truth was, Ruru and I had already become this intimate. Being involved with my cute little sister felt somewhat embarrassing, and maybe I should have stopped things here. But there was no time to think. Ruru's tongue slickly teased my nipples, and I couldn't help but moan out loud. "Someone might hear us," she whispered, sealing my mouth with a deep kiss, then smoothly began an unprotected insertion with practiced skill. As the bed creaked beneath us, she grinded her hips, and I gasped, "No... I'll make noise..." before releasing inside her. At that moment, my sleeping friend suddenly stirred, forcing Ruru to hide under the futon. While chatting with my friend, she secretly reached over and teased my sensitive spots. When my friend finally left, Ruru whispered softly, "Onii-chan, let's do it again." By now, our relationship had grown this deeply intimate.