I've come to the conclusion that it's down to personality (an obvious one), age gap & sex. From watching differences between my siblings & I, ex's siblings & my kids.

My older girls will hopefully get along as adults. They're too close, yet too far apart in age as kids/teens. Add to that, dd1 is competitive & self centred, dd2 just wants to be like her big sister & loves everyone. They get along when dd1 has convinced dd2 to do/give her something she wants :/

The younger 3 aren't too bad. They fight like siblings, but they manage to play together as well. Dd3 likes to stir the pot, so fights are still frequent & sometimes pretty epic, but they're are much less significant when they get along the rest of the time.