Known in English as "Engaged to the Unidentified". A 4koma series by Arai Cherry which received a 12-episode anime adaptation.
Kobeni Yonomori is a high school girl who lives with her mother and older sister Benio. Upon turning sixteen, Kobeni is shocked to learn that, due to an arrangement by her late grandfather, she is engaged to a boy named Hakuya Mitsumine who, along with his little sister Mashiro, come to live with Kobeni. As Kobeni spends time adjusting to this new family, she learns there is more to Hakuya and Mashiro than meets the eye.
