Your particular problem can be solved by your wife telling her family her wishes as well.
Better yet, she can write them down and give you AND her family a copy.
That way, everyone's on the same page, so to speak.
BTW, if she is found unconscious just after she decides to divorce you, all bets are off and her parents get her.
My wife has told me that she doesn't want to be kept alive in a PVS and trusts me to make the decision. I've said the same to her. That is what marriage should be. But if "conservatives" have their way, that doesn't matter until we get our family's approval, and possibly have an independent review.