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 bed earlier than her older sibling. How many times do you hear the words, "It's not fair!" or "You love her more than you love me."?

I know you're waiting for the magical secret to eliminate sibling rivalry in your home. Sorry - there are no magical secrets, but here are ten practical suggestions to help minimize the tension between siblings.

Avoid making comparisons of any kind - not to your children and not at any time they may be privy to the conversation. Focus on the gifts in each of your children and capitalize on their differences.

Encourage your children to express their resentment or angry feelings. This does not mean you allow them to scream at or hurt each other. Teach your children to handle their anger constructively, not destructively. Acknowledge and validate your children's feelings so they feel understood. Help each child see things from the other child's perspective.

Have very clear boundaries about personal belongings and personal space. Make it a rule that no one is allowed to use another person's belongings without permission. Teach your children to respect each other's personal space - bedrooms, their bodies, etc. A child needs to feel like there are at least some things that belong to him.