Tomodachi no Oneesan to Inkya ga Koi wo Suru to Dou Naru no ka?

A self-proclaimed unattractive gloomy high schooler, Reiji Hiramiya, every time he goes to visit his close friend's house, he gets messed around by his older sister, Sakuya. Drinking his drink as she pleases, barging into the bathroom, and crawling under his blanket in the middle of the night... Thinking that perhaps he has done something to make her dislike him, he asked his close friend whether he has any idea, but he only gets a reply in a incomprehensible way, "You still haven't noticed it, huh." On a certain day, Reiji loses in a game and has to listen to three of Sakuya's wishes. Reiji goes along with it even though he has doubts, but could he gradually begin to understand Sakuya's true feelings...? This is the story of super-dense youth and a can't-be-honest older sister until they become a lovey-dovey married couple.