You need:

A Durable Power of Attorney

A Health Care Proxy

A "Living Will" which outlines your desires in case of terminal illness.

I suggest (having needed all of these documents and being very grateful that they were in order) that you see an attorney on behalf of your family member and explain the situation. The attorney won't like it much but will make sure you have the proper forms for your state of residence and that you know how to complete them. They will, of course, have to be witnessed and perhaps notarized.

This is nothing to mess around with as the time when you need these documents is exactly the time when you absolutely must have them in order.

You will probably want to have several certified copies of the documents and the attorney will help you with that as well.