Osananajimi No Ijou Kawaii Imouto-Chan

Osananajimi No Ijou Kawaii Imouto-Chan summary:

Yanagi, a second-year high school student, has a faint crush on Mizuki, the younger sister of her childhood friend Momo.One day, while rummaging through Mizuki's personal belongings in her room, Yanagi discovers the truth about Mizuki...?