While I agree with all that's been said about bickering, can I just post a word of warning about bullying?
I spent my childhood in terror of my brother because my mum wouldn't get involved. My dad was away a lot and mum insisted we were to sort it out ourselves. In actual fact, I was beaten up on many occasions. My brother learned he could throw his weight around by threatening me and that it was his right to hurt me; I learned that bigger, stronger males would always win the argument and (from my mother) that I deserved to be hit.
Not really messages to teach children, I think: "she made me angry, I couldn't help it, if she hadn't argued with me, answered back, it wouldn't have happened. She deserved it." where have we heard those excuses?
Eventually, my brother beat me up so badly, he messed up my face and our father finally saw the black eye, swollen face, split lip and we both learned new lessons. My brother learned that, if he ever hit a woman again, there'd be hell to pay from someone bigger and stronger than him; I learned that I wasn't on my own, expected to take whatever was thrown at me.
Incredibly, I can still remember my mother being angry with me and blaming me for getting him into trouble.
I just wanted to say, fair enough that they should learn to sort out their own differences but do make sure that one isn't victimised by the other. In my book, if someone throws punches, that immediately puts them in the wrong. They need to learn that physical abuse is never acceptable.
Sorry if I've wandered off topic a bit, but I do think it needs to be said.