My sister, who attends an all-girls school, is incredibly pure and always asks me, her older brother, when she doesn't understand something... But today, she suddenly showed me her breasts, touched herself, and asked incredibly erotic questions with a serious face! It seems she's troubled by the physical changes of puberty... Before I knew it, my sister had grown into a mature woman, and I accidentally got an erection! Then, my sister, with a serious face, became very interested in my erect penis! Pretending to give a serious sex education, I ended up going all the way with my sister.