'Only children' are behind in this aspect when they go out into the world. A quote from Andrew O.C.(Only Child) Wong, in his article, On Only-Child-dom, 'I'm irresponsible, oblivious to others, apathetic, and I screw around too much. Of course, if I was born the eldest, who's to say that my personality wouldn't be different? In fact, I'd have probably turned out differently.'
Tips that will help you from going gray when dealing with sibling rivalry:
As teens get older, direct mediation is not required.
Actually, if you step in to their argument, it's going to make it worse. It reinforces fighting as a way to get your attention. Of course, if your teenagers are screaming at the top of their lungs, or hurting each other, looking the other way isn't going to do much good either. That brings me to my next tip.
Set the rules beforehand.
The most important rule to set is on physical violence. Make it clear to your children that physical violence is not allowed. Teen's that would never even think to touch another person, are usually the ones who will take the first swing at their brother or sister. Set up consequences before the act occurs.
Spend some time alone with each child.
Do something that they like. Everyone has their own talents and interests. Take the time to bring these out in each child. Try not to make one child's interest more important than the others.
Give Them a Forum.
If your teenagers know that there is a time and a place to air their grievances, they will use it. We use family nights after dinner during dessert. They need to air their grievances with respect to their sibling, but we do hear them out. No problem is too small.
Recognize cooperative behavior.
If your teenagers are able to work out a problem, take notice and give some praise. This will reinforce cooperation and help with future 'battles'.