Set up a “cooperative area” or “neutral zone.” In this area, all children are obliged to obey the nonnegotiable rules of play: respect each other, talk reasonably, listen to each other, and do not push or hit. The zone is magical, and all must follow its enforced ethic of “peace on earth and good will toward all.” If a child can’t follow the rules of this tiny magical kingdom, he must leave the area until he can follow directions. (Make sure that the best toys are located in the cooperative area.)
Give siblings spaces that are completely their own, especially if they share a room. Let them know that if they need alone time, they can go to their special place. This “special place” can be a tiny nook somewhere in the house (e.g., a closet, a corner, etc.), as long as it is only theirs.
If things are really tough, delegate chores. Give them each their own “job”—in separate areas of the house, preferably—and put them to work.