I understand your difficult position, and how scary it is to think about joining a new family that may not always understand or support your priorities (
whatever they may be). However...welcome to marriage!
They call them "out-laws" for a reason.
I think if their attitude (and your fiance's way of dealing with them) really bothers you, NOW is the time to really take a close look at the family and really decide whether or not you'd be able to happily make the compromises that would allow you to become a part of them. And if not, you're right, this could spell trouble for the future marriage.
Or, maybe this one area is the ONLY blotch on an otherwise potentially great relationship with them.
I'm sorry, I'm giving "advice" again, when all you really want is sympathy!
Sorry, it's the "mom" in me.
Well, I do for sure feel for your frustration and disappointment.