How can I break my aunt's terrier mix of her aggressive behavior towards other dogs?

My aunt's terrier mix is very aggressive towards other dogs and has even attacked them. I'm going to be dog sitting and doing some basic training but I don't want her and my anti social/non aggressive Chi-Weiner to clash.I do plan to keep the girls separated and my aunt doesn't tolerate the behavior. She doesn't know what to do so she just doesn't take her around other dogs. In our family we usually bring our dogs to our family gatherings but Sox always has to stay home.

I doubt you'll be able to do it if the dog is always aggressive to other dogs. Some terriers are naturally dog aggressive, especially if not socialized young. Working with a trainer might work, but it will take a long time (not necessarily your time frame), may not work, or the dog may not be 100% reliable. I'd plan on keeping the dogs separated.

Your Aunt will have to do this sweetie. You can be firm and let her know you yourself will not tolerate her crap behavior, But if the dogs owner doesnt care and thinks its cute, it wont stop ever.

Get Leslie McDevitt's book Control Unleashed. It's designed to deal with dogs who "lose it." The general term is "reactive dog." This is a dog who reacts to his environment and loses control. Leslie's book is superb. It's very hands-on, applies to a lot of different situations (from dogs who get nervous or over-excited and to those who are fearful and become aggressive). It's a superb book and among professional trainers and/or agility handlers, her book is quite popular even though it's only been out less than a year. I use a lot of what she covers in her book myself with my dog.A quick summary of approach that the book takes: dogs with low thresholds respond to stuff in the environment (a squirrel which causes them to lose focus and run across a road with cars in order to chase the squirrel, a dog who gets so excited at an agility try he has no focus and just blindly races around the ring, a dog who sees another eyeing him or a dog barking excitedly and behaves aggressively by going after the other dog). This book shows you how to raise the dog's threshold so the same stimuli doesn't set them off. Instead, they're able to maintain control and focus.It's a better approach than using other old-fashioned approaches like squirting water or using a choke or even prong-collar. Those approaches don't get the dog to calm down, they just try to overwhelm the dog's feelings with stronger counter-feelings or sensations. A far better approach is to find a way so the dog doesn't react and lose control. That gives you a better behaved dog with no bad side-effects.