Mamahaha no Tsurego ga Motokano datta  

My Stepmoms Daughter Is My Ex, 継母の連れ子が元カノだった

It seems that mizuto irido and yume ayai were once lovers in middle school. they flirted with each other but eventually disagreed on trivial things and became more irritated with each other than excited. eventually, their relationship ended when they graduated from middle school. despite this, the two of them met again in an unexpected way. it is not immediately clear whether mizuto irido or yume ayai is older based on the information provided. however, it seems that their parents remarried and had children who were ex-lovers. the former couple decided to follow a "sibling rule" where whoever became conscious of the other as the opposite sex would lose. despite living under the same roof and sharing memories from the past, mizuto irido and yume ayai struggle to disregard each other when they encounter each other in the bathroom or commute to and from school together.