Lori Radun posted an article on the internet dealing with sibling rivalry.  Here are some of the suggestions she offerred.  Go to the link and read the rest of her ideas.

..."the most significant factor that affects sibling rivalry is parental attitude. As parents, we know we should treat our children equally and fairly. And most of us probably try very hard to do that, however inconsistencies will still exist. There may be a child you get along better with because of your personalities. Perhaps one or more of your children are easier to handle so they have a tendency to receive more loving treatment from you. Children pick up on every bit of inconsistency and they don't always understand why things are different for each child. Older age children have more responsibilities, but more independence. A younger child just thinks it is unfair that she has to go to ...   more »