The only way you can win these kinds of battles is not to fight them. if Mom and Auntie behave badly and you behave well and fairly this will not be lost on your offspring. I know that you already know this.
As far as helping your kids get through it. Listen, commiserate, honor their anger with their mother, but never join them in it. Express your confidence in their ability to cope with whatever comes.
Sorry that they and you have to dance this dance... it is such a waste of peoples energy. I'm particularly sorry that the Auntie feels that its her place to stir up a situation that is complicated enough under the best of circumstances.