Nanaco robin manga

Nanaco Robin summary:

Yoshino Nako has a saying. "If I'm happy, I can do anything!" For her sister, Nako-chan steals the groom of an arranged (political) marriage and helps them elope. But when the Hayami group starts falling, she must pay the consequence by housing the groom's rumored delinquent younger brother... From Fushigi Mystery (cont.) The younger brother, Konatsu, seems to be glared by all students who sees him. He used to be a leader of a gang [as rumors said]. The two, Konatsu and Nako, were rumored as couples and starts a crazy life living together in one house. How will the progress move?!