Kid dynamics.
Ages of the kids and their own developmental phases.
All ages, they get tweaked. Hence fighting.
The younger ones do what they see. Younger kids are parrots.
The Eldest though, cannot solely be held accountable. Because they are also, frustrated as well.
All I know is, my 2 kids fight too. But as a whole get along.
BUT... I make sure that I teach them, that if at ANY time, one of them wants alone time... all they have to do is tell me and ask and tell the other sibling. Kids get burnt out too. Of always being around each other.
Also, if bored, kids get on each others nerves more.
Teach them to use their WORDS, not fists.
Teach them about child development. I do with my kids. I simply explain, to my Eldest, how her little brother is developing, what he can or cannot understand or be expert at yet and what his age phases are. I explain to to my youngest, how he is who he is and his sister is older and different. I explain, these things to them.
They are not the same.
They are different ages.
This helps.
I also teach them what a sibling is.
What a "family" is.
How we are all a "TEAM."
We look out for each others back.
But kids do need, time to themselves too. Even if in their room. They do not have to, always be interacting. And, kids get tired. Then they get fussy when tired or hungry. Then they tantrum or fight when they are over-tired and at the end of their rope.
Kids have limits too.
Teach them to recognize their limits and 'boundaries.' Teach them to say it. How to say it. The words for it. Role-play with them.
Practice.
But yes, kids fight.
So expectations has to be, in line with their age and development.