Sibling Rivalry CNN interactive http://cnn.com/HEALTH/indepth.health/parenting/children/rivalry.html

So your children don't always get along. In fact, sometimes they seem to genuinely despise each other. Should you be surprised? No. Is there anything you can do about it? Maybe.

First off, think back to your childhood.

Maybe you had siblings. Did you always like them? No, we didn't think so. Or maybe you were an only child. Would you really have liked being forced to share your toys and your parents with a little brother or sister? Sure, maybe sometimes, but ALL the time? Not likely.

As long as there are brothers and sisters there will be sibling rivalry. But that does not mean you have to sit back and watch your children torture each other. We've surveyed some of the most popular parenting sites on the World Wide Web, looking for advice on how to cope with sibling rivalry. Here's a summary of what we found.
Guiding Principal Number one
Being Fair

Much of the literature on sibling rivalry seems to stress one principal that parents and children should try to keep in mind: All things are NOT equal. Sure, you should try to be fair, but don't think you have to count out the peas and make sure each child gets the same number at dinnertime. Let your children know that the decisions you make are based on need, not necessarily on fairness. At the same time, try to avoid using the phrase, "life isn't fair." To a child, those words often translate into "Mom isn't fair" or "Dad isn't fair."
Guiding Principal Number Two
Taking Sides

The experts also seem to agree on another point: Don't play referee. If your kids try to get you to choose sides in a dispute, refuse. Let them try to work it out on their own. You can teach them negotiating skills later when things calm down. With a little practice, they may learn there are perfectly good alternatives to playing tug-of-war with a disputed doll.
Create the Right Atmosphere

The key to keeping peace in the house is preventing fights before they break out. Here are few preventive measures you might try.

Set Ground Rules

Sometimes you need to tell your kids that if they fight, they'll have to pay the consequences. Some suggestions:

Sources: