I know this sounds rediculous, but when my sons got into rip roaring fights when they were little (now 28 and 25), I would tell them, "You need to be good to each other because your brother's the only one who can give you a kidney if you need it someday." This would diffuse the situation, and they'd usually stomp off to their respective rooms. Recently my oldest son told me that really had an impact on him. What I thought was just a silly thing to say to get them to settle down, turned out to have a true impact on my son's behavior and made him stop and consider how important what they were fighting about really was.