In an emergency, a doctor's decisions could override your living will for another reason; if proper care requires you to be treated in a certain way or if there is an ethical obligation, that could nullify your wishes. But his family can't annul his living will. They cannot take away your authority to create your own treatment and care plans. In fact, you always retain the right to override your own decisions.
You can also allow your appointed representative or power of attorney to modify the terms of your living will or revoke a directive. A living will lasts until you cancel it. You can change your mind after signing a living will. If you want to cancel your living will, you must break your copy and notify other people (such as family members and doctors) that they also have a copy.
They are usually done by those who are terminally ill or are nearing the end of their lives. Getting your document effective immediately will not give your agent the authority to override what you want in terms of treatment; you can always order your own medical care if you have the ability to do so. However, when there is an emergency, a doctor can annul his or her living will to ensure adequate care in a certain way and within ethical obligations.