His ex abandoning SD at Christmas… again!

Sorry I need to have a long rant as my anger is preventing me from having my morning nap. My fiancé has a 10 year old daughter with his ex-wife. We have a 5 year old son and 8 week old daughter. Fiancé and ex are supposed to alternate Christmas, but she’s been with us for the last 2. Her excuse last year was that her 2 other daughters were with their dads, so would just be her and SD. SD is mostly a sweet child, we’ve had our issues but it’s got better as she’s matured. My issue is that her mother has just announced that she’s travelling to her home country for Christmas. She was just there in the summer for almost 3 weeks - she gave us 3 days notice before this trip! When fiancé asked why is she going again, she said that her life just can’t be about being a mum and she needs a break. I understand that it’s hard being a single mum. She has 3 kids with 3 different men and all 3 fathers see their daughters regularly. She chose to be a mum and now just abandons her children. She used to go and not tell the fathers, leaving her daughters with her sister, but her sister’s husband got tired of this. My fiancé would also be upset when he found out and go and pick up his daughter. The worst part about all of this is that she said this in front of SD. How cruel! She hasn’t spent Christmas with her mum since 2021 and now she hears her mum complaining about how much of a burden she is?! My heart breaks for the poor child. I can’t imagine not seeing my two on Christmas Day. It’s going to be tough keeping a 10 and 5 year old occupied throughout the break, whilst dealing the sleep deprivation from my newborn. His ex has no regard for our household (she sent her daughter over one year when she had COVID) and doesn’t even seem to care for her own child. We will just have to find some cheap or free activities for the kids to do and spend some time doing arts and craft, baking and watch movies at home. We will take them to the Christmas @ Eltham Palace (I highly recommend it to anyone who is local), but any other suggestions for days out are most welcome!