It sounds very much like she wants to escape everything. Not an adult thing to do when you have responsibilities, but we can still feel that way.
The very best thing for you to do is back off and leave her alone. Focus on your children and giving them as good a time possible for Christmas (under the circumstances). Detach from her, but don't be mean or cold. Just go ahead and proceed with Christmas and act as if she's a cousin staying over.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!
