Why siblings fight so viciously?

There were four schools (Hairesis) of Philosophy in ancient Greco-Roman civilization, The Academy of Plato, Lyceum of Aristotle, The Garden of Epicurus and Stoa (Porch) of Zeno (Zeno of Citium). These schools were originated in same Hellenistic civilization but from different Philosophers. Those of us who are not so young know that the Communists and Capitalists were battling, competing and refuting each other very viciously though these both politico-economic ideologies were born mostly in Britain (Though Karl Marx was educated in Germany –University of Bonn and University of Berlin- but he spent from 1843 till his death -1883- in London and used British museum as resource to develop and support his “theories of surplus value”…. Similarly, although Capitalism has not very known origins but father of Capitalism, Adam Smith (known for his famous book, “An Inquiry into the Nature and Causes of the Wealth of Nations” was a Scottish Political economist and Philosopher who was educated in University of Glasgow and University of London… so we can say that both Capitalism and Communism were largely born in Britain). If you do know that how communists and capitalists were battling each other very bitterly then you have to just extend the same level of bitterness to the two schools of Philosophy in ancient Greco-Roman Civilization, Stoicism and Epicureanism. In fact all four schools of Philosophy were battling each other but these two have well recorded rivalries. Coming back to our modern time people who reads academic papers sometimes wonder, why academicians battle to refute each other so viciously? Or for those who do not read academic papers but watch news may think, why Sunnis and Shiites are battling so bitterly and murderously despite both claim to be Muslim and in principles have no differences? Or even closer, sometimes people wonder, why siblings compete so bitterly with each other (Though not all ;)…

The first reason that might come in our mind may be commonality. Because those Philosophical schools, ideologies, religious sects, siblings, colleagues are so much in common that they have very small things or very little things to fight over and it would only make sense if they fight very aggressively, very viciously or murderously to make those battles look important.
Well, Dan Ariely (who is famous for his bestselling book, “Predictably irrational” …This website allows to download it for free, if you are interesting to read it, http://search-ebooks.eu/p/predictably-irrational-pdf... ) has an interesting explanation for it (though he is not talking directly on the same topic),

He says that people who are very similar with each other affect drastically the image or position of each other by influencing on others’ judgments and decisions. This is really interesting and somewhat counterintuitive. For example, if two persons or things have all in similar then a small difference though negligible but highlighted, will influences the decisions drastically. In other words, people who are very different try to project their commonalities while people who are similar try to project their differences. Following is Ariely TED talk. If you watch it till end, I am sure you will be amused…