My sister, a high school girl, is very innocent, and if there's something she doesn't understand, she always asks me, her brother... Today, she suddenly showed me her tits, touched my vagina, and asked me some super erotic questions with a serious look on her face! She seems to be worried about the physical growth that is typical of adolescence... Before I knew it, I got an erection from my sister's body, which had grown into an adult woman! Then, with a serious look on her face, she was very interested in my erect penis! I pretended to be a serious sex educator and ended up having sex with her.