Which is she saying?
a) My wish is to be buried at Site A, 500 miles from here, with or without you (because that is where my family is, or the place I feel is “home”, or whatever)
b) I don’t care where I’m buried, as long as it is 500 miles from whatever cemetery you choose.
c) Please do not bury my remains in the same cemetery with your parents, who never liked me or anywhere else near this town, where I only moved to make you happy (or some other personal problem with your site of choice)
If it is (b), that deserves immediate further discussion. Happily married people do not hold sentiments like that. Even if it is ©, while she may never change her mind about where to be buried, she may need some help in letting go of some resentments towards your family, she may be harboring a deep wish to move that you aren’t aware of, or the like. That deserves attention.
If it is (a)–and I’m guessing it is–then you have some choices:
a) Decide that it is more important that you be buried with her than exactly where you are buried, and agree to buy two plots (or three, or whatever is required to accomodate her sister) at her site of choice.
b) Figure that it will not matter after you die whether your earthly remains are buried next to your wife’s, and decide that it is more important to be buried at your own cemetery of choice or your “home turf”, and stipulate where you are each to be buried in your will.
c) Opt for cremation, and stipulate in your will that her remains and yours are to be moved to your mutual site of choice after you are both gone and neither can visit the temporary gravesite of the other (which would be in a columbarium in a local cemetery). I suggest cremation because it is unduly difficult and sometimes not even practically possible to disinter, move, and reinter full-sized caskets many years after burial.
d) Decide to wait for the time to come and let the survivor do whatever he or she sees fit with the remains of the first of you to die.
At any rate, I think it is a good idea to talk to your pastor about how you might discuss the matter further with your wife.