Mummiehunnie so much can trigger abuse memories and pain - for me it's the snow right now and heck if the man who had his way with me would send me a card I would go spare.
They may well be sending it to still exercise control, but you don't need to let them. Cut or shred the cards and put them in the bin. You do not owe anyone anything.
Try to make a space in your house that is safe - maybe that has nothing christmas related in it but something that makes you feel safe. My safe things are a teddy bear I can hold with flashbacks and my mobile phone with texts from my friends in it.
It doesn't matter what works for you - do something positive that is good for you and makes you feel safe