My Stepmom's Daughter Is My Ex - S01

Listless geek Mizuto Irido and introverted nerd Yume Ayai seemed like a match made in heaven, brought together by their shared love of literature. Sadly, their differences slowly grew, and they broke up right after graduating middle school. But as if by some divine comedy, the two end up reunited as step-siblings. A rivalry starts to build between the former couple, with neither willing to admit the other is the older sibling. To "settle" things, Mizuto and Yume agree on a rule: whoever crosses the boundaries of what counts as normal for siblings loses, and the winner not only gets to be called the older sibling, but also gets to make one request. But now that they live under the same roof, the memories they still share begin to shape how they act, possibly rekindling feelings that might not have been completely gone in the first place.