Karin is a caring little sister who lives with her two older brothers. Both brothers have girlfriends and get along well with them, but they have unique troubles: the first brother can't insert properly during sex, and the second brother is a virgin who doesn't even know how to interact with girls. Seeing her brothers struggling, the kind-hearted sister offers erotic guidance. They start with sumata (thigh sex) since they are siblings, but the aroused sister accidentally lets the cock slip inside. Surprisingly, both brothers are quite skilled!! Although they were only doing sumata for various reasons, they can no longer hold back and proceed to full insertion!!