I sympathize with your situation. Before I was married, I generally spent Thanksgiving, Christmas Eve, and Christmas Day hours away in another state with my family. Now that I am married to a man who shares custody of his two minor children with their mother, my life — of necessity — must revolve around the child-custody schedule.
This was a major anxiety for me prior to committing to spending my life with him, because I knew that the custody arrangement does not permit either parent to have the children for the entire holiday on Thanksgiving or Christmas. Instead, both parents are required to split the actual day of the holiday, which makes it impossible to travel (my family lives about five hours away) until at least half of the holiday is over (so we can’t arrive until evening), or, if, we have the children for the first half of the holiday, we have to wait until the next custody period, which is several days after the holiday, and travel then. This situation definitely has meant a significant adjustment for me and my family of origin. (On the plus side, DH’s parents live in the same city as my parents, or I would have no idea how I would ever be able to be with Darling Husband on or near a holiday and still have a chance to see my family.)
Although only one of your FI’s children is a minor, and he or she will only be so for another year, it appears that your Fiance still plans to spend every Christmas Day with his children. Unless, as prior posters have suggested, he and his children are willing to share that time with you and your family, I do not see a clear path forward for you to be able to spend Christmas with both your Fiance and your family, unless your family is willing to come to you.
Although I agree with PPs that your Fiance is not being fair to you, he ultimately is free to choose to spend the holidays with his children instead of your family of origin. However, you also have every right to determine that this is unacceptable to you and that you cannot move forward with marriage under these conditions.
There is one thing your FI should consider. His children are not very young, and soon they will be wanting to spend some of their holidays with THEIR SOs families. When that happens, your FI would be alone. Does he really want to risk losing you now, over this issue? Or, instead, will he be willing to consider some type of reasonable compromise (every other year, taking the kids to spend the holiday with your family or every year traveling, with or without the kids, to be with your family the day after the holiday so that you could still your family)?
I hope you are able to find a reasonable compromise.