Koukou Debut shita Juumai ga Mount Tottekuru.  

My Younger Cousin Who Made Her High School Debut Asserts Her Dominance, 高校デビューした従妹がマウントとってくる。, Koukou Debut Shita Itoko ga Mount Tottekuru.

My younger sister is three years younger than me. Initially, she was just an ordinary girl, but after entering high school, she transformed into a wonderful person. Although I used to believe that she would always dominate me in our daily lives, there were times when tears appeared and it turned out that her appearance had changed while her personality remained the same. It seems like we have become a pure older brother-younger sister love comedy pair.