Sibling rivalry is a nightmare, even my 16 year old boy will continuely say I love my daughter more (who is 14) if he does not get exactly what the other gets, for example if she needs extra money for school project or something and he doesn't need it, that's enough to get his jealously up, been doing it for years, it's going to be a problem in the future, and more importantly it will bring resentment to me and the wife for his mythical thinking.