Although rare, it's possible your dog suffers from a neurological condition causing aggressive reactions to other canines. (dogcare.dailypuppy.com)
You'll definitely want to prevent all the first sibling rivalry you can, especially unexpectedly or unintentionally aggressive reactions. (whattoexpect.com)
It's never too late for your dog to learn to deal with his fears and other triggers for aggressive reactions. (straightpoop.com)