Zo is extremely dominant. She is also the lowest on the ladder. Whenever one of any of the others who are significantly older has a health issue, she goes for them.
The answer is boundaries at every turn. Make it absolutely clear every single time that you, the one in charge, values the older, weaker dog and you're setting the rules. It is not up to our Zo to set the rules, ever. In fact, every time we come back from a surgery or a medical procedure, I make it clear in no uncertain terms that no one is to come near the dog who came back from the vet, even to sniff, especially if she or he is coming out from anesthesia and absolutely cannot put up a defense.
I raise my voice. It can sound harsh. But Zo knows without a doubt that she is not to go after the other dogs. It's a question of trust with the older dogs as well. Fran actually got much closer to me after her latest surgery as a result and it doesn't diminish my relationship with Zo. Dogs need boundaries.
Our behaviorist went through a horrible scenario when she was raising Springer Spaniels because she decided to let her pack sort it out but I don't want to upset you. It made me really think though and we're a successful household as a result. So, basically, be firm at all times.