"When I grow up, I'm going to marry you, Onii-chan!"

This kind of promise is the least likely to be kept in the world. At least, the protagonist, Yunagimoto Yuusuke, believed as much when he thought back on the distant memory of that promise.

Then, one day, a younger female relative of his showed up at the apartment where he was living alone and said "I'm old enough to get married now. Will you keep your promise and take me as your wife?"