Saikin, Imouto no Yousu ga Chotto Okashii n da ga.

High-school girl, Kanzaki Mitsuki, just got a new older brother, when her mother remarries a man with a son. Soon after, her new step father gets transferred overseas and her mother follows him there, leaving Mitsuki alone with her new step brother, Yuuya. Feeling abandoned by her mother, she stays distant from Yuuya, never really getting along very well with him nor even keeping a conversation with him, until one day....