If your sisters don't want to heal this rift, I agree with others, there's nothing you can or should be expected to do.

As for Christmas, I feel the same way. Found out, a few days ago, that my sis-in-law's older daughter + partner + her son are all staying there on Christmas Eve. Apparently all arranged sometime ago. All my brother has said to our mother is it's up to you what you do at Christmas. What a load of bull***t!

Sorry, but if this rift healed between your mother and sisters, then there'd be another one. You'd probably find yourself stuck in the same position again. Cynical I know but if your mother wants to sort out this problem, for health reasons or not, she should do it on her own.