My stepmother's burden is my ex-girlfriend.

A couple became boyfriend and girlfriend in a junior high school. They had a sweet time together, but then they misunderstood each other over a small matter, and their anger and frustration outweighed their initial attraction... Later, they broke up when they graduated.
However, the two, who are about to enter high school, Mizuto Irido and Yume Ayai, reunite in an unexpected way.
"Of course I'll be the older brother." "I'm the older sister, is that even a question?"
The child of her parents' remarried partner turned out to be their ex-boyfriend whom they had just broken up with!
The ex-lovers, mindful of their parents' feelings, agreed to abide by the "sibling rule": "Whoever mistakes the other for someone of the opposite sex loses." However—they unexpectedly bumped into each other again after showering, and were alone together on their way to and from school...
The memories of "that time," coupled with the situation of living under the same roof, made it impossible for the two of them not to pay attention to each other's every move!