There are several different parts to your question, so let's take them one at a time.
You ask about HIPAA disclosure forms. A HIPAA form is a way for a patient to give written permission to health care providers to discuss and give information, to the person named in the form, about the patient's medical condition. A specific HIPAA form is not absolutely required, but a doctor or other health care provider must be completely certain that a patient gives permission before releasing patient information. Without specific consent to release information, a doctor or other provider may only give out minimal information to assist family members or others in caring for a patient -- for example, instructions on how to care for a patient upon being released from the hospital, or signs to watch out for when a patient begins taking a new drug. However, if a patient tells a doctor or other provider not to give out information, the doctor or provider is bound by that request.
As for decisions about treatment, a patient can refuse either a specific treatment or all treatment for a particular condition, or any treatment at all. Neither a family member nor a doctor has the right to force a competent adult to undergo medical treatment. Sometimes the key term with this issue is "competent." In order for a doctor or other health care provider to override a patient's decision about medical treatment, the doctor must be convinced that the patient is not mentally competent -- either temporarily or permanently -- to make that decision. In that case, a doctor is supposed to honor the decisions of a person whom the patient has legally appointed health care proxy in a document called an advance health care directive, medical directive or living will. If there is no valid medical directive, the doctor is supposed to make a decision that is in the best interests of the patient; doctors usually will discuss this with immediate family members if a patient is clearly unable to make any decision himself.
If a patient is able to communicate a specific decision about treatment, a doctor will not override it unless the patient has been definitively diagnosed with Alzheimer's or other dementia and is clearly incompetent to make a decision. Unless such a diagnosis of incompetence has been formally entered in the patient's medical records by more than one doctor treating the patient, or has been determined by a court, a doctor is very unlikely to go against a patient's wishes, no matter what the family wants.
If you believe that a loved one is incompetent to make health care decisions but the doctors do not agree -- or privately agree but are not willing to make the decision to override the patient's decision -- then you will have to consider conservatorship or adult guardianship proceedings. This involves going to court and having a judge declare that the person is no longer competent to make his or her own medical treatment decisions. If the person you are seeking guardianship over is still able to communicate his or her wishes about treatment, you almost certainly will need a lawyer experienced in guardianship or conservatorship proceedings to handle this with you. Be aware that this can be an expensive, time-consuming and emotionally very difficult process.