Due to my father's remarriage, I started living with my new stepmother, and a teenage girl came into our home as my new stepsister. She behaves politely and shows a gentle, calm demeanor in front of our parents, but with me, she holds nothing back. She often comes to borrow money or play little pranks, constantly causing me trouble. Then one day, I accidentally discovered her in the changing room, pulling down her panties and masturbating, which left me completely shocked.