Hi, I'd like to help you with your question.
It sounds like your fiance's family want to do things their way. And if it doesn't go their way, they get upset and refuse to compromise. This is very self centered behavior and it is very natural that you would feel shocked and upset by it. Most people are willing to compromise, especially if they are invited to someone else's home.
When you invite people to you home, you are only expected to be a gracious hostess. By that I mean you make them comfortable, offer them a place to sleep and meals if needed, and you treat them with respect. But your invitation does not require you to give up your home and allow them to overrun your holiday traditions. This is your home, not theirs.
Unfortunately, their point of view seems to be that it's their way or the highway, at their home and yours. This puts you in a bad position. They are being inflexible and not offering to make this easy at all.
A good way to approach this would be to have your fiance deal with his family. He should talk to them, explain that you are happy to have them in your home and that you will be following your family's Christmas traditions. If they refuse to come, then he should let it go.
However you indicated that your fiance is siding with his family. This makes it very difficult for you. Although he should take your side in this situation, he has not so you will have to make the choice of what you want to do. Offering to compromise on some things is a good idea, but not on your family's traditions. Let them know you are sticking to your family's holiday traditions. Let them know you will assist with any compromises they need to make (sleeping elsewhere for instance) but you will not be changing your plans. Then let them decide if they want to come over or not. It is up to them to decide if they can handle being good guests. Giving up your family's traditions will only allow them to feel they can bully you anytime. Then it will set a precedent for years. Stand your ground now and let them see you deserve respect as well.
I hope this has helped you,Kate