POA (Power of Attorney) are never irrevocable as the person who granted the POA retains the power to amend or revoke or establish a new one, unless the person is fully demented. That is the same for Advance Healthcare Directive. So, the last executed POA or Healthcare directive is effective and controls NOT the prior documents, assuming your mother had presence of mind and was lucid and capable of understanding the documents she was signing and their legal effect.