So sorry about this, Notjustaprettyface! It must be exhausting! I don't agree that you should keep reassuring DD, not to the same extent, anyhow. IMO, whenever she complains, I would just remind her that you've told her you love them both the same - and then change the subject.
As for her saying DS "got away" w/ doing less, I would have pointed out there's no rule about what you "have to" do for family or how much, if anything at all on Xmas - especially not on Xmas. But the moment is past, of course.
One concern.. . Did you and DH compliment DS but not DD? Have there been other incidence where DD felt you praised her brother but not her? Even if only when they were kids? (Sometimes adult sibling rivalry is a leftover from childhood sib rivalry, as I'm sure you know.) I've seen situations where the parents fell into the habit of focusing on the achievements of one child over the other's (maybe b/c that one child was struggling at some point, etc.) One of my friends is CO from her DD and family, partly b/c she always sings the praises of her DS while barely noticing DD's accomplishments. I get the impression it's b/c her DS has yet to find lasting love, lives alone, and my friend sees him as "lonely," etc. IDK for sure, but I do know she fails to see that she does this, let alone how much it hurts her DD. Could you and/or DH be making a similar mistake? If so, please be more careful about this. But still, please don't get drawn into any silliness like comparing celebrations, etc.