I feel for you Spadger. I wonder if you feel worse than they do?
Christmas is a challenging time for many families. If you feel like going away, versus the potential upsetting vibe at Christmas, then go ahead.
At first I thought, you had already accepted daughter (a) so go ahead as was planned.
Or, you could discuss with daughter (a) and (b) and suggest you have everyone at yours.
On the other hand, if you simply do something for yourself and go away, perhaps this will give them a wake up call. You can simply say that you do not wish to be in the middle of all the squabbling at Christmas time so you are taking time out for yourself this year and perhaps it will be back to normal next year.
Follow your heart.
Fx