C.P.
You need to teach them to communicate effectively. This means that conversation is respectful and has a give-and-take. One person listens, repeats back what they've heard to ensure they understand, and speaks a response.
Most importantly, children should know that we should be MORE respectful and appreciative of family than anyone else. If they are not obeying that House Rule, they should be removed from the situation until they can obey.
It doesn't matter why they're arguing. Disagreements are going to happen. It's HOW they're arguing. Teach them differently.
You can't stop the behavior if you are just telling them "stop doing that" or "stop hitting/yelling," but you never actually affect a change. Stop the situation. No matter who seems to be at fault, stop the situation and put them physically away from one another in different rooms.
Don't ask what happened. It doesn't matter. The problem is the disrespect they have for one another, not what they're actually fighting about. So tell them each that they aren't even allowed to talk to one another unless they can do so respectfully, without physical violence.
Do this each time they're in a fight. It works, and you will see changes.