Can a member of the family ignore an Advance Care Directive?

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).