When my girlfriend came to my house to play, my younger sister Yuri came with me and was very interested in the "sexual act" that we learned in class. However, Yuri's breasts have grown so much that they are surprisingly large even for an adult. However, she is insensitive to the attractiveness of her own body and uses her breasts to innocently seduce me in order to experience the "sexual act"... After we crossed the line, she started coming to my house regularly and begging me for the "act of pleasurable sex"...