BlessedArt
She asked if her son and DIL told me they planned to go. I answered yes of course. She asked me in a highly annoyed tone if I were serious. When I answered yes she stated well she’ll make alternative plans, whatever that meant. When I asked if she were joking there was a long awkward pause and then she abruptly ended the call. Hasn’t returned a phone call since and we speak almost daily.
Just a possibility, but she may have fooled you and actually been immature this entire time.
It's always easy to act mature when things are easy; when you're in control. But if she's not used to hearing the word No (perhaps when her son was smaller and didn't have the brains and/or brawn to say No), than she may truly feel like this common-sense boundary between 2 adults is actually a wholehearted attack on her (ego).
If she is actually immature, than she may truly struggle to consider the future implications of her actions. This irrational behavior in adults is usually easily explained away as some trauma-response (usually childhood), and there isn't room to think about the future under traumatic conditions.
Your best bet to solve this quickly, before the holidays is to explain her behavior is irrational and borderline abusive (I know what it's like to have a caregiver give the silent treatment as one way to enforce compliance, it doesn't feel nice). Explain to her the future consequence of her short-sighted actions, because she may really have not thought this completely through.