Mamahaha no Tsurego ga Motokano datta

Mizuto Irido and Yume Ayai ended their relationship at the close of middle school, and both expected high school to begin without ever having to deal with the other again. Then their parents marry, leaving the former couple with an embarrassing new reality: they are now stepsiblings living under the same roof. For their parents' sake, they agree to behave like a proper family, but their old rivalry quickly returns, especially over which of them deserves to be treated as the older sibling. They create rules meant to keep them safely within sibling boundaries, yet shared memories and daily closeness make those boundaries increasingly difficult to define. The feelings they thought were over may not have disappeared as completely as either wants to admit.