Wow, thank you everyone!
I didn't expect the categorical support! I really thought there'd be at least a few opinions saying to just suck it up for the festivities.
After reading all the comments on here I felt brave enough to call my family and tell them the real reason why I wasn't going to be available on Christmas. I spoke to my Mum and she told me that everyone knew something was wrong and was concerned. But now it makes sense to them. And they were sympathetic about the situation. Which is quite a relief.
Regarding a few other posts. You all are right, it was very difficult for my SIL to tell me she was pregnant. She knows what we've been through and was visibly upset while telling us. I feel bad that she feels bad, but I also appreciate her consideration for our feelings. It does not make the situation any easier to deal with, though. It still hurts so much. I still feel guilty for not being able to celebrate it with her, and feel guilty for avoiding her, because we are close.
Most of our immediately family know about our TTC issues, to some degree, but some family doesn't know. But I've decided that I'm going to take some crazy pills (valium) and put in an appearance tomorrow... for the sake of Christmas.
DH and I will just quietly disappear if it gets too difficult.
Thank you everyone, for showing me that a lot of people do understand this situation.