Engaged to the Unidentified

Season 1 Episode 8 - Eps 8: Only a Little Sister Can Soothe Sadness over a Little Sister

A typical girl, kobeni Yonomori, finds out which she is betrothed to Hakuya Mitsumine, a boy the exact identical age . Hakuya starts living with her, attracting his sister who's a fundamental school student with him also moves in from the countryside.