Known in English as "Engaged to the Unidentified".  A 4koma series by [[Arai Cherry]] which received a 12-episode anime adaptation.

[[yonomori kobeni|Kobeni Yonomori]] is a high school girl who lives with [[yonomori_akane|her mother]] and older sister [[yonomori_benio|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 [[mitsumine hakuya|Hakuya Mitsumine]] who, along with his little sister [[mitsumine mashiro|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.
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