This is a confusing question - if your father is deceased, why is the power of attorney she may have as his agent relevant? A power of attorney ends at death...unless you are referring to a transaction performed while he was alive and she was his purported agent. I would defer to a TX attorney but powers of attorney are not always recorded - sometimes they are if the agent is dealing with real estate for the principal.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
Mr. Zelinger is correct; all powers of attorney cease to be effective automatically when the grantor (or principal - the person giving the power to someone) dies. Your step mother may have the paper that granted her (or appointed her) as your deceased father's attorney in fact; but it is no longer effective.
Mr. Philo limits his practice to matters concerning real estate. By answering this question no attorney-client relationship is established. The answer is a general answer to the question posed and is based on the facts and circumstances stated in the question.
As has been said, a power of attorney ceases to have any effect upon the death of the person who signed it. But, that doesn't stop someone from trying to use it and if they business or person they show it to doesn't know that the principal is deceased then she might be able to use it. It would be fraud if she did. Further, in Texas the law does not require that a power of attorney be recorded with with county clerk to be valid.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.