The Christmas scene is the last time these girls have time to celebrate with each other and be family before it transitions to backstabbing and getting caught.
A movie cannot be a Christmas movie unless Christmas plays a central mechanism in the transition of the narrative from A to B.
So Die Hard is clearly NOT a Christmas movie because Christmas changes nothing in the story.
The Christmas scene is the last time these girls have time to celebrate with each other and be family before it transitions to backstabbing and getting caught.