From a recent Time Magazine article :
Siblings have a socializing effect on one another," says psychologist Daniel Shaw of the University of Pittsburgh. "When you tease out all the other variables, it's the play styles that make the difference. Unlike a relationship with friends, you're stuck with your sibs. You learn to negotiate things day to day." It's that permanence, researchers believe, that makes siblings so valuable a rehearsal tool for later life.
Thanks to Spunky for the link.