Kiss de Okoshite.

A high school freshman named Ena frequently receives unusual love confessions. Her friend Miu suggests it might be due to some kind of curse preventing her from experiencing mutual affection. When Ena's mother remarries, Ena learns she will have a younger sibling, but the supposed "little brother," Touma, is actually her age and attractive. As they are not related by blood, Touma warns Ena not to fall in love with him. The story explores how their relationship evolves as they become part of the same family.