I have awaken to find WORLD WAR 3 (as witnessed by the amount of hair tufts on the floor) had taken place down stairs. And sometimes during the course of the day I will hear a bit of loud screaming, often not knowing who is getting the short end of the deal. If it is not happening 24/7 then it sounds like normal sibling rivalry. If it escalates, I would try to find out if something in their routine has changed. Stress from an outside source could be straining their relation.