Ann Finkbeiner explains why each sibling tends to mark off territory, which psychologists call "sibling deidentification":
The idea is that siblings need to avoid competition. Sibling competition is nothing to take lightly. Sibling colonies of bacteria secrete antibiotics that kill each other off. The first African black eaglet out to hatch of its egg pecks the second one to death. A quarter of spotted hyena pups are killed by their siblings. The siblings are competing for their parents’ resources and attention, of which the parents have only a certain amount. So the siblings are operating in a closed system, one pie only.
But Cain and Abel aside, we humans can’t go killing off our siblings, so we agree to go our separate ways, we deidentify, we stake out separate niches. … Heather’s brother is good at finances, so she can’t be. And here’s the charm of it: a closed system might be lethal for competitors but it’s golden for cooperators.
(Photo via Awkward Family Photos)