Koukou Debut shita Juumai ga Mount Tottekuru.

I have a younger sister who's three years younger than me. Before, she was just a generic girl, but ever since getting into high school, she's become a wonderful gal. Going through our everyday life, she would dominate me, or so I thought, but there were tears, and believing lies, huh? It's only her looks that have changed!? Dominating and getting dominated! A pure older brother and a bad and easygoing little sister, a 1-on-1 love comedy begins!! (Source: Kadokawa, translated)