No.  An Advance Care Directive is a legally binding document.  If a family member is appointed as a Substitute Decision-Maker they must also follow the instructions in the Advance Care Directive.  The exceptions are that the Substitute Decision-Maker cannot:
Make a decision which would be illegal, such as requesting voluntary euthanasia.
Refuse food and water to be given to them by mouth.
Refuse medicine for pain or distress (for example palliative care).
Make legal or financial decisions (unless you have also been appointed as an Enduring Power of Attorney for financial matters).