There is no such law. But such a document is a good idea. You can die testate, that merely establishes certain guiding principles and laws. It doesn't mean anything else.
In this case, the closest relative (the husband) gets to decide. If Terri told him her wishes, the court is LEGALLY and MORALLY obliged to follow up on those instructions, Greer-bashing anarchists notwithstanding.
I would prefer my wife to handle my affairs if I became brain dead (although I have Living Will on file, partially as a result of this case).
And yes, Terri was brain dead.
No, she wasn't. Please watch the video. You can't go in and out of being brain dead, so the claim that they took hours and hours of video just to get 8 minutes of her being alert is hogwash. Just watch the video, and you'll see:
http://www.freerepublic.com/focus/news/1381701/posts?page=181
Post #174.