Kiss de Okoshite.

Ena is in her first year of high school and she keeps receiving confessions from weird guys. Her friend Miu jokingly says "Maybe you have a curse that forbids you to have a mutual love?" When Ena's mother decides to remarry, Ena discovers that she'll have a little brother and sister... However, the "little brother", Touma, actually is the same age as her, and he's handsome! Since they don't have a blood relationship, Touma says: "You absolutely can't fall in love with me, okay?" How will their relationship develop, now that they have become a family?

(Source: Shueisha, translated)