NYH-155 - Always showing off her panties in a mini skirt, my stepsister and I had sex while the family was out

My father remarried and I suddenly got a stepsister. Living together was fine, but she always wears a mini skirt and shows off her panties. I was caught peeking, and now she teases me by shoving her butt in my face. When the family left us home alone, she asked for a massage. As I grabbed her big butt, she got excited. Since her panties were fully exposed, I was rock hard. The tsundere sister suddenly turned proactive and taught me various things, so I went all out with my teenage energy.