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As the oldest of NINE children, you'd think I could relate to this topic. Soooo not true. I honestly have a great relationship with my younger sibs. Most of them are ten years or more younger. But there isn't anything I find more frustrating than dealing with my own kids' sibling rivalry. Why can't they just have one big love fest?? Okay, so maybe I'm being naive. Here are some sites and tips that I find very useful in the matter.
One thing I'm learning is that you have to try and stay out of it. This site has good tips.
A great book: http://www.amazon.com/Siblings-Without-Rivalry-Children-Together/dp/0380799006/ref=sr_1_1?ie=UTF8&qid=1325084002&sr=8-1
My local library had a copy I was able to check out.
Good website for "dos and don'ts." http://childdevelopmentinfo.com/parenting/sibling_rivalry.shtml