Responses
Um he was introduced into the family quite late, well after she already started to like him as a fan. Even if this was actually treating two situations differently, it would be unfair to expect her to just automatically stop liking him plus it's not as if they grew up together as siblings. It was their PARRNTS' decision to get married later in life. So that would be the ONLY way they're treating the two situations you mentioned differently.
GUYS, I KID YOU NOT. YOU WOULD NOT THINK THE PINK GIRL IS GONNA END UP WITH THE RED BOY RIGHT??? the step brother thingy is literally on the cover AND the title. i wouldnt have guessed it otherwise, like... they're really there from the start.
Still tho, like arent they impulsed that its wrong that the one youre calling "brother", having both your parents together married and both of you calling mom and dad together... like this just makes that your step sibling will never be your real sibling which is honestly so off puting cause whats the point of adoption too then? i really cant like this kind of thing. but im not dumb enough to think the mangaka will change her mind ☠