I am unsure of when to separate religion from business and am confused about why it's ok for her to take the vacation days designated for Christmas, yet she can't cater to the needs of the business.
I think you should ask her this question. Let us know what she says too.
My mother in law was a cake decorator and a Jehovah's witness. She would have been out of a job if she refused to do holiday cakes, or birthday cakes. There aren't too many other reasons people get decorated cakes is there? It seems to be up to each individual as to how much they will do at holiday times.
You could be right that she is using it as an excuse since she is willing to take the days off that Christmas and holidays allow. Maybe you could ask her to work those days and consider her reaction.
Most of my Jehovah's witness friends who work in hospitals, volunteer to work on Thanksgiving, Christmas and New Years so their co workers can be with their families. Double time pay too.