What rights as a child of a parent who has a spouse who has placed them in a nursing home due to dementia.

If you are not your father's agent under either his Medical Power of Attorney or his Durable [Financial] Power of Attorney, it seems unlikely that you can override your stepmother's decisions.  When there is no Medical Power of Attorney, the Texas Health and Safety Code directs health care providers to turn first to the spouse for directions.  When there is no Durable [Financial] Power of Attorney, the Texas Estates Code allows a spouse to seek appointment as a Community Administrator, handling the couples' community property.  If your father has significant separate property (and he might, since this is apparently a second marriage) and lacks legal capacity, you can seek appointment as guardian of his estate.  The Texas Estates Code gives preference to the spouse but that preference can be overriden by a showing that she is either robbing him or not acting in his best interest.  To pursue this further, you might want to talk with a local member of the National Academy of Elder Law Attorneys (www.naela.org) or seek an Approved Guardianship Attorney on the State Bar of Texas website.  Please remember that many practice in counties other than that in which they are listed.