Body language is a good indicator IMO. I think your best course of action is to keep calling her out when she mumbles, makes noises and talks through the dog. Do so in a polite but firm way. Eventually she will get the message and stop or you will grow tired of her games and decide you don't need this kind of crap in your life anymore.
I would not initiate any work or planning for the party. Let her initiate then you join in. She is very used to getting catered and coddled when she wants something so this is a GRAND opportunity for you to do a 180 and let her put in some effort first then you join in. If she wants to prepare for this party as a team she must behave like a team player and not simply a delegator.
How you handle the party preparation is a huge opportunity. And how you handle the actual party is also huge. Just because her family is present it doesn't mean she won't be called on snotty comments, noises, mumbles and so on. You must be *very* consistent in this or it will never change.
