Mamahaha no Tsurego ga Motokano Datta

When they were in middle school, Mizuto and Yume were a perfectly typical couple.
Bouncing between flirting and bickering over trivial matters, they eventually decided to break up shortly before entering high school.
However, it was upon learning of their respective parents' remarriage that the former lovers realized they were going to be reunited again...
To avoid spoiling their parents' happiness, they accepted the situation and established the "siblings' rule," according to which the first one to feel attraction towards the other loses!
But... it's hard to act normally when the frequent run-ins in the bathroom and the commutes to and from school together bring back old memories! (source: animotaku.fr)