My sister Mami had little knowledge about sex and was troubled in her relationship with her boyfriend. To comfort her and listen to her concerns, her younger brother agreed to help. However, since Mami had never seen a penis up close, her younger brother allowed her to observe his. This caused both of them to become aroused, leading to handjob, blowjob, and eventually ejaculation. From that day on, the younger brother became her practice partner. Although they knew their sibling relationship was wrong, Mami justified it by claiming it was for the sake of her boyfriend, becoming increasingly submissive. Meanwhile, driven by his desire to have sex with his sister, the younger brother used every method to pleasure her until finally crossing the line.