I would recommend getting out your video camera and secretly taping their violent activities. Play it back to them...they will have a good time. I am yet to hear of young siblings who follow Buddhism to the B. Fighting, mischief, rivalry are all part of growing up. Don't worry your head about it. They are doing just fine.

Assert yourself when things get too ugly (i.e. a mini blood-bath), else it is better to give them space to resolve their own issues between themselves. The younger one is naughtier because he is still too young to understand what right and wrong behaviour is all about.

Your 9 yr old most certainly has a higher degree of maturity and understanding. Plus, she's a girl...aren't most girls less violent than their brothers ;) And a 5 yr old boy isn't exactly supposed to be prince charming.