I would also get your older girl a senior checkup. Frequently a younger dog will take the top position over when an older dog becomes ill. Dogs know if another dog is ill long before humans do and smart owners will recognize that and have their dogs seen when another dog starts acting more aggressive toward the older dog. So I would have that done as well.
I did want to address the shock collar as well. Using the tone is ok, but I would avoid using the shock. When a dog is displaying any type of aggression toward a person or a dog and a shock is given, the dog tends to feel that the dog or person was the source of the shock and it actually makes it worse. The instruction book that came with the shock collar should have mentioned not using it for aggressive situation.
I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .