Holiday traditions can range from casual to sacred. Casual traditions are usually flexible, while the sacred traditions carry much more importance.
If a specific meal is more of a sacred tradition than a casual tradition, then I'd ask the rest of the partcipants before I'd change anything. In our family, the meal's very much a secondary part of the Christmas celebration, so the hostess chooses the menu. On the other hand, the Thanksgiving turkey is an automatic "must have," and anyone who would like to serve duck instead would first want to talk to the others.
It sounds like the family in the OP places significant importance on the Christmas dinner menu, so I think the hostess shoudl try to accommodate those items, even if she asks others to contribute.