Jitsu Wa Imouto Deshita. Vol. 1 (Light Novel)

Ryota Majima is a second-year high school student who is excited to have a new sibling in his step-brother Akira after their parents remarry. The two hit it off, and soon Ryota comes to adore Akira and enjoys always spending time playing together. But when Akira asks Ryota if he likes him, he realizes there’s been a huge misunderstanding and that Akira isn’t his brother, she’s actually his sister! A new romance comedy between a high schooler and his younger sister who acts like a younger brother begins!
Notice: This book is written in Japanese.