When I sneaked in, I thought my step-sister, who was born from my parents' remarriage, was doing something in my room, and I found her masturbating with the legs of a kotatsu in her pussy! I was so surprised by the sight that I couldn't take my eyes off her, and my step-sister found me, creating an awkward atmosphere... She lectured me about masturbating in someone else's room, but I couldn't convince her at all because my crotch was completely erect! I don't know if she got horny or embarrassed, but she grabbed my dick and asked me to teach her the right way to have sex...