Shishunki Kyoudai

Yousuke Hino is thinking about when to start using "ore" instead of "boku." Recently, he's been worrying about the fact that he can't beat his twin sister Mikage at anything. In height, grades, and sports, Yousuke feels like she has him completely beat, and as a result has become uncomfortable around girls. He wanted to change things since becoming a second year middle school student, but...

(Source: Wani Books, translated)