Imouto no Otetsudai

While trying to borrow a dictionary; Nana -the younger sister- catches his brother doing.. well; you should know by this being a +18 book. At first thing are very awkward between them, but suddenly the brother started asking favors as to "even things out". Before long "borrowing a dictionary" became some sort of keyword for them. "Original" doujinshi by the circle TAKANAEDOKO.