First of all, Welcome to PC! I hope you find that this site is very helpful and the people here are very kind.

I am so sorry to hear that you're fiance and sister are not getting along. and I agree with what Lee is saying about it being your parents house, not your sisters. They should be the one to make the rules, but I also understand that they want to be able to see their niece. But it definetly isn't fair that your fiance is missing his sons first Christmas. I think you need to talk to your sister and explain to her why this is so important to him, and even if she can't forgive him, at least be okay with letting him be present at the holidays. After all, family is very important. & do the same thing with your fiance, get him to realize that your sister is important to the family too, and that this all needs to be worked out, or just put aside for now, and enjoy the holidays together as a family. , Like you said, the holidays are for the kids, and even if they can't forgive eachother, they can at least pull it together and act like mature adults.