Hi K.. It's good to know that it's not just my kids who seem to fight and be mean to each other all the time!
When my older 2 get into a bickering spell, They loose the privilege of certain tv shows and games. They are only allowed to watch educational tv and play educational games. I've noticed that they tend to play what they watch and if they are watching more "social-skill-building" or non-violent shows (such as "A Baby Story" on Discovery)and playing non-competivive games such as Big Brain Academy or Nintendogs, they are more calm and play better together.
I've also noticed that when they are together too much, without a break from each other, or are trapped inside due to weather, they get restless and irritable with each other. Sometimes, the only thing I can do is separete them and have them play by themselves and they learn to get along out of bordem of playing by alone.
We also started something that eliminated a lot of arguments. Rach was born on an odd month and Tyler an even month. So on odd days, it's Rachael's day, and even day's, it's Tyler's. Their days determine who can finish off the cereal, which tv show to watch, who gets to sit next to the baby in the van, which movie to rent, etc. Anything that they once bickered about is now settled by whose day it is. And they both know it, so they don't argue about it. (on months when there are 31 days, the 31st is "share day" and they need to compromise or I get to choose)
Just some suggestions. The last one I took from a Parenting magazine article. I wish I could take credit for that one.
Good Luck,
J.