possibly because they didnt spend childhood together. I think picking fights is natural for children. As they grow up they are used to it so we continue to bicker with our siblings over little things. However the twoin the story spend their childhood missing each other. I think they both feel like they owe each other something.
Age difference also helps. I have 15 years difference with my big sis and 13 with my big bro. We never ever had a fight and we love each other... I totally relate to those two I hate how siblings always seem to fight or hate each other in manga...
did they have a large age difference? I thought it wasnt that big? but I don’t remeber...
they are sibling and they get along well?? h- how? me and my brother always pick fights for no reason lol