Answer:
You will need the assistance of a lawyer. She should be able to fill you in on the details.
Obviously that is a poor answer. I am a lawyer and can tell you that, generally speaking, you will need the assistance of a lawyer to actually have the relative declared imcompetent. That said, this doesn't mean that people shouldn't be given the answer or the criteria, to simply say "get a lawyer" is not helpful and furthers the notion that the law is unknowable or too erudite for common folk. If you have the answer, give it. If you don't, don't.