Mamahaha no Tsurego ga Motokano datta

My Stepmom's Daughter Is My Ex follows Mizuto Irido and Yume Ayai, two former middle school classmates who once bonded over a shared love of literature before their very different personalities pushed them apart. After graduation, both assume that chapter of their lives is over. That changes when their parents marry and they suddenly become step-siblings under the same roof. The new household is awkward from the start, and neither of them is willing to concede the role of older sibling. To keep their pride intact, they set an unusual rule: the first one to cross the line of proper sibling behavior loses, and the winner gets to make a request. But ordinary routines make distance impossible. Every meal, errand, and conversation drags their shared past back into the present, forcing them to confront the embarrassment, stubbornness, and half-buried feelings they would rather ignore. As Mizuto and Yume try to survive family life, their rivalry becomes a constant reminder that the past may not be as finished as they hoped.