My Stepsister is My Ex-Girlfriend

Mizuto Irido and Yume Ayai, both literature enthusiasts, had a brief relationship in middle school before separating due to their differences. They're now reunited as step-siblings. The two, still harboring feelings for each other, establish a rule to resolve the issue of who's the older sibling: whoever breaks the norms of sibling behavior loses. As they live together, shared memories start to resurface, potentially rekindling lingering emotions.