Ore no Imouto ga Konnani Kawaii Wake ga Nai

Kyōsuke Kōsaka, a normal seventeen-year-old high school student, has not gotten along with his younger sister Kirino in years. For longer than he can remember, Kirino has ignored his comings and goings and looked at him with spurning eyes. It seemed as if the relationship between Kyōsuke and his sister, now fourteen, would continue this way forever. One day however, Kyōsuke finds a DVD case of a magical girl anime entitled Hoshikuzu Witch Meruru (星くず☆うぃっちメルル?, Stardust Witch Meruru) which had fallen in his house's entrance way. To Kyōsuke's surprise, inside the case is a hidden adult video game titled Imōto to Koishiyo! (妹と恋しよっ♪?, Let's Fall in Love with Little Sister!). Kyōsuke attempts to fish out the culprit who dropped the case by bringing up the topic of magical girl anime at the family dinner table. All that comes out is a strong negative reaction from his parents, especially his anti-otaku policeman father.

That night, Kirino bursts into Kyōsuke's room and, in perhaps the first conversation she has initiated with him in years, says they "have things to talk about." Kirino brings Kyōsuke to her room and shows him an extensive collection of moe anime and lolicon bishōjo games she has been collecting in secret.

Source: Wikipedia