I would definitely suggest exercise first and foremost. A tired dog is a happy dog, and usually too tired to fight.
It does seem odd that it's very sudden. Is everyone spayed? Any of the girls coming into heat? Is Bette in good health? I've seen a few times where a sick dog will begin getting attacked... because they are "weaker" and so the attacker senses that.
So I guess a vet check may be in order for both Piper and Bette, IMO, would be my first step. Then I'd work on it as a training issue.