Here’s a few links to help parents resolve sibling rivalry issues:

6 Ways to Prevent Sibling Rivalry: http://proactiveparenting.net/articles/6-ways-to-prevent-sibling-rivalry

How to Stop Sibling Rivalry With Your Kids: http://www.metroparent.com/daily/parenting/parenting-issues-tips/stop-sibling-rivalry-kids/

How to End Sibling Rivalry: http://www.positiveparentingsolutions.com/sibling-rivalry