Karin is a caring little sister. Both of her brothers have girlfriends and get along well with them, but they suffer from a unique problem: they don't know how to have sex properly. Brother (1) has a girlfriend but struggles with the act, often failing to actually insert his cock. Brother (2) just got his first girlfriend and is a virgin who doesn't even know how to interact with girls. Seeing her brothers' struggles, the kind sister provides erotic guidance. They practiced using thigh jobs to avoid the awkwardness of sibling sex, but as Karin became more aroused, she accidentally let them insert. To her surprise, the brothers were actually quite good at it! After various reasons for sticking to thigh jobs, they eventually couldn't hold back and went for the full insertion!
