Shiiba-san no Ura no Kao with Imouto Lip

In a certain normal Japanese high school, there are two peculiar students: a lonesome boy and Shiiba, a mysterious girl. Neither of them have many friends; but unlike the dull and boring boy, Shiiba is beautiful and considered to be in a vastly different league to the other males in school. Therefore, no one has had the courage to ask her out. Nonetheless, it comes as a pleasant surprise to the plain boy when Shiiba invites him to an empty room. Elsewhere in the world, a certain teenager has just returned home from a long day at school. As he approaches his room, however, he hears sounds from a porn video he had once watched. Startled, he charges into his room and finds his younger step-sister, Miku, watching the tapes. It is no surprise to her that teenage boys are interested in these things, but what she does not expect is that she would take an interest in it herself… [Written by MAL Rewrite]