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No one, including Cary, as yet seems to have addressed the LW's problem. Instead they are all moralizing about Christian vs Christian-lite, etc.
Tell the family members that while you invited them to the event you did NOT invite them to be your houseguests. You can send or e-mail them information on reasonably priced accomodations nearby, and that is the end of your duty. I also have no idea why you would spend one moment of stress worrying about their wants for the ceremony. Are they footing the bill? If not, there is NO discussion, NONE. Demand some respect and you'll get it. And LW also never mentions the fiance after the first paragraph. Does he have an opinion here?
If the level of communication is as bad as it seems from the letter, heaven help the marriage (if they get to that point).