If the health care directive (health POA) does not say she has power to dispose of his property upon his death or manage his assets, then your stepmother has no power to do so. If she holds a durable power of attorney for asset management and it specifically says she can manage his finances and dispose of his property via will, then yes.
I recommend you have an attorney review the powers of attorney.
This is not legal advice and does not intend to create an attorney-client relationship. You should consult an attorney if you have any questions regarding this matter.
California does not recognize Living Wills. Could there be a living trust? Some trusts can be changed by the non-incapacitated spouse depending on the language of the trust. No one is entitled to notice of a change to the living trust, however, that doesn't mean she is not engaging in improper behavior. You really need to see an attorney. Feel free to call me at (619) 656-88478 to discuss further.
No legal representation exists by virtue of this answer. It is recommended that you contact an attorney directly for a more complete response.
Your question teeters on several different topics. Your father's living will, is likely an Advance Directive for Health Care. However, it is also possible that you are referring to a living trust for your father (and step-mother). As for whether your step-mother can change your father's will, the answer is no, only your father can do that. At this point it seems as though there is much confusion. I would suggest that you gather whatever paperwork you have and consult with a local estate planning attorney for fact specific direction.
** LEGAL DISCLAIMER ** My response above is not legal advice and it does not establish an attoreny-client relationship. When responding to questions posted on Avvo, I provide a general purpose response based on California law as I am licensed in California. In reviewing my response, you are specifically advised that your use of, or reliance upon any response I provide is not advisable. I do not have all relevant background details or facts related to your issue / matter, thus I am not in a position to give you legal advice. Further, your review, use of, or reliance upon my response does not establish an attorney-client relationship between us nor does it qualify as a legal consultation for any purpose. For specific advice regarding your particular circumstances, you should consult and retain local counsel. Law Offices of Eric J. Gold www.EGoldLaw.com Telephone: 818-279-2737 Email: email@example.com