I Kissed My Girlfriend’s Little Sister is a Manga/Manhwa/Manhua in english language, Comedy series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.nl fastest updating comic site. The Summary is

Hiromichi Sato has been dating Haruka for a month, and their relationship is quickly heading in a good direction. But one day Hiromichi’s parent announces they’re remarrying and he’ll be getting a new step-sister. When he meets her, he’s shocked to see that she looks exactly like Haruka. No worries, though — they’re totally different people. They’re just twins, each one living with a different parent since the divorce. Certainly nothing to worry about. It’s not like there can be any wacky misunderstandings brought about by living with your girlfriend’s identical twin, right…?