It's a dorama of 10 episodes rather funny, adapted in 2008 from the manga of the same name ( done by Hijiri Chiaki ).

15-year-old Yoko, a young girl who is constantly tormented by her self-centered and devilish older sister Makiko, who works for a government office after having graduated from a famous university. But despite her ill nature, Makiko's actions tend to somehow make things better for those around her, causing others to praise her as an "ally of justice."

From d-addicts wiki

I liked this dorama, the big sister is really odd and sometimes we want to know... isn't she a genius who just like to seem mean? And what has the poor little sister to do is fun x]