Advance directives, living wills, medical powers of attorney, HIPAA releases, Do-Not-Resuscitate orders, etc. have limits. These are written documents that cannot express all your wishes. The best of them, the medical power of attorney, appoints someone to make your decisions if you are not able to make them. All of them can be fought in court. However, if you have chosen the proper person for you medical power of attorney, that person will stand up for you and make sure your wishes are followed.
If there is disagreement within your family about end-of-life decisions, then frank discussions of your wishes are imperative. You do not seek to change the mind of those opposed to your decisions, only that they will honor them. If, in the worst case, it is clear that your wishes will not be followed, then I would put something in writing to the effect that this person is to be excluded from any health care decision pertaining to me. Be sure to sign, have it witnessed and better yet, notarized. Depending on how serious the conflict, this might be an issue to discuss with an attorney.
Advance directives only work if the hospital and the doctors have them. I recommend that a copy be made of each document and kept in a plastic bag in a readily accessible spot, e.g. taped to the door of the refrigerator, in the refrigerator freezer, in the top drawer of your desk, etc. The person with your medical power of attorney should have a copy of all documents and know where the originals are kept. Don't keep originals in a safety deposit box unless the person who holds your medical power of attorney has access to that box. Even then, I think is is better to keep these documents in a safe place in your residence.
Each time you go to the hospital, even if you were recently hospitalized in the same one, have copies of your documents to be put in your chart or records. Make sure that all your doctors are aware you have executed these documents, and most importantly, communicate your wishes directly to the doctor. Doctors will honor your oral directives if at all possible.
Advance directives work if they have not been done in a vacuum, that is, done on paper but never discussed. I have used directives to make decisions about the end of life successfully. The lack of advance directives can make a difficult time even more horrendous. Ease your loved ones burden and have these documents done.