My Stepmom's Daughter Is My Ex

Mizuto Irido and Yume Ayai were once the perfect pair-a quiet bookworm and a withdrawn nerd, bonded by their shared love of stories. But their differences eventually drove them apart, and they broke up right after middle school. Fate, however, has a strange sense of humor: they’re suddenly reunited as step-siblings. Now forced to live together, they refuse to accept the other as the elder sibling. To settle the matter, they make a rule: whoever breaks the unspoken boundaries of siblinghood loses, and the winner gets to be called "older sibling" while making a demand. Yet, as they navigate their new living situation, old memories resurface-hinting that the emotions they thought were gone might still be smoldering beneath the surface.