As I said in response to your other question: Anyone can sue anybody for anything - whether or not the suit would be successful is impossible to predict. However, to play devil's advocate for a moment, I can see how you might be held at least partially responsible - although your dog was on a leash, your sister knew the other dog was acting aggressively and yet she did not move your dog away. If she is too young to make that determination, then she is too young to have been walking your dog.
Given how this has turned so ugly so quickly, I suggest you speak with a local attorney for guidance. The State Bar of Arizona can give you a referral: www.azbar.org/ You may also need to notify your homeowners insurance carrier.
If we do not have a signed fee agreement I am not your attorney and this is not legal advice.
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