From FairGame (JCafe24):
It’s about a boy named Nishizawa Kazuya who is in 2nd year of middle school. Unlike his younger sister, Rika, he is good at a lot of things. He is also the fastest runner in the school. His parents and other people think that he is a responsible and hard working child. He seems like a perfectly good child but he has a secret that he can’t tell anyone. Every day he enjoys peeking at his sister changing clothes.