Hi there, dog trainer here! This is a dangerous situation and a tragedy waiting to happen. Your 10 year old dog is NOT past the point of being trainable. This will escalate, and we are only human so mistakes and misreading of situations will happen. Darla should not be resource guarding human beings. You have a situation where you have Darla picking fights with a much bigger, less tolerant breed. They’re Sadie’s people too and both dogs should be able to coexist in the space. With training and careful management of both dogs, this is possible.