Mamahaha no Tsurego ga Motokano datta

A high school student moves there hoping to escape a failed relationship, only to discover their ex is now their stepsibling. The student is now constantly encountering their ex both at home and in school, including in the same class. The ex insists on being seen as the older sibling, but the student refuses to back down, asserting their own position as the older brother in the new family dynamic. Despite maintaining a friendly sibling facade for their parents, the two acknowledge that their past romantic feelings have faded, revealing incompatibility. The relationship is now strictly familial, with no return to what it once was.