"Too naive and submissive to older men!" I developed desire for my girlfriend's younger sister and secretly had sex with her. (Iwai Ruka, SONE-681)

I was in charge of tutoring my girlfriend's younger sister, Ruka. Although she was still a middle school girl, she was innocent and adorable. She had very little knowledge about sex and often carelessly exposed her underwear or wore a see-through bra. And her breasts were quite well-developed. Even knowing she was my girlfriend's sister, I still treated her as a "woman." Before I knew it, I was kissing her soft lips. Ruka looked at me in surprise, but I was captivated by her tongue. This was probably her first kiss. Because of her limited understanding of sex, she hadn't fully grasped the situation. She didn't show any aversion. She was so innocent… Realizing the despicable nature of my actions, my penis was already erect.