Ore no Imouto ga Saikou no Okazudatta is a Manga/Manhwa/Manhua in (English/Raw) language, SUGGESTIVE series, english chapters have been translated and you can read them here. You are reading chapters on ManhwaZ fastest updating comic site. The Summary is
My very own little sister caught me playing with my meat toy to a little sister Eroge game, then my cool, calm and collected little sister suddenly decided to help me play with toys... She even made my friends play with toys, and somehow got the most beautiful girl in school involved!? Will i be able to get a girlfriend if i can only koom to my little sister!? and also... Why is she still watching me kooom!?