Ore no Imouto ga Konnani Kawaii Wake ga Nai.

Synopsis

While being treated by Kirino after getting his face wounded by some reason, Kyosuke reveals to her that he and Ruri decided to start dating, and much to his surprise, she does not complain about it. In their first date, Ruri wears a special cosplay she designed specially for the occasion, much to Kyosuke's astonishment. As the day pass, Ruri shows more about her tastes and preferences to Kyosuke and she reveals that it was written in a special guidebook made by her in order to attain her wish, which according to her is to have him and Kirino get on terms with each other. Despite not understanding the meaning behind her plans, he agrees to help her with it. The two then spend the following days hanging out together at their houses, with Kyosuke being introduced to her younger sisters in the occasion, and by the end of summer break, they go to a festival together. Knowing that school is to resume in a few days, Kyosuke claims to Ruri that he will always treasure the moments they passed together during the summer, while she is about to take the next step of her guidebook, which is to break up with him.

(Source: Wikipedia)