Read other letters about this article
Great insight from Cary, and I think Psycprof gave some helpful suggestions if the relatives absolutely must be barred from the house at all costs, but mostly I agree with the suggestions that having family in the house is no big deal, the ceremony itself does not sound like it is being hijacked, and learning to deal with relatives you don't "get" is part of being an in-law.
In addition to not seeing what the big deal is with the family, I don't get what makes the siblings "dysfunctional" (unless that's shorthand for saying she doesn't like them and doesn't want to try to understand them) or how she differentiates between her future husband's parents, who she sees as soon-to-be "inlaws," and his siblings, who she does not. Guess what? Those "dysfunctional" people will be your in-laws. Brothers-and-sisters-in-law, to be specific.
Sounds to me like she wants little to do with his family beyond the parents. Also sounds to me like that's an impossible goal unless she very rudely excludes them from her life and all future religious/holiday festivities.