Please humour me, I am an only child so I have no experience of this to draw on. My husband works long hours so he's not really much help as he's rarely here.
We have 4 DCs, they've all rubbed along pretty well until recently. However, DS1 (8) and DS2 (7) have become so bad with each other that they now cannot be left together unsupervised at all. Literally not even to go to the toilet. DS1 has ASD and has always been difficult, DS2 has been amazing at putting up with it but increasingly he's not dealing with it at all. The fighting is always physical, this morning they were pushing each other as soon as they woke up because one dared to sit next to the other. 
Most fighting is stupid stuff like that or, worse, Lego. They argue, then fight, about Lego constantly. "That's mine, you stole it." "HE'S BEEN IN MY ROOM." Blah blah.
Is this normal? How do I deal with it? I want to leave them to it but if I do DS2 ends up getting hurt, DS1 pretends to be hurt to get DS1 into trouble and then everyone is miserable. DS1 is bloody hard work at the best of times but his attitude currently is causing all this and making himself and everyone else miserable.