Ore no Imōto ga Konna ni Kawaii Wake ga Nai (English: My Little Sister Can't Be This Cute ).
The story follows Kyosuke Kosaka, who discovers his perfect, "model" younger sister, Kirino, is secretly a hardcore otaku obsessed with adult "eroge" games. oreimo-plus
To understand the parody, it helps to know the source material it satirizes: Ore no Imōto ga Konna ni Kawaii Wake