Hmmmm - difficult one. I don't think YABU but it may depend on the circumstances.
My DH's ex is single and has no other dc's so if me and my DH were to have dsd for xmas day, then she would be on her own (well, with her family but no dc's or dp). We have 2 other dc's so, whilst we would love to have dsd for xmas (and actually suspect at some point she may want to spend a xmas day with us and her half siblings), at the moment, she has spent every xmas day with her mum and her family.
We do have her for 5 days before and we spend that with her paternal grandparents etc. We hand over at around 5pm on xmas eve.
FWIW, my parents were divorced and I used to have xmas lunch with mum and xmas tea with dad. DH's parents were also divorced and he did alternate years and I believe that, all things being equal, a court would tend to grant alternate years