Saikin, Imouto no Yousu ga Chotto Okashiinda ga.

A high school girl, Kanzaki Mitsuki, just got a new older brother when her mother remarries a fellow divorcée with a son. Soon after, her new stepdad gets transferred overseas and her mother follows him there, leaving Mitsuki alone with her new stepbrother, 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—