Our Dad's in a nursing home. Can my stepmother legally deny us access to him?

The Nursing Home will respect the rights of wives over anyone else. So will the courts. Now begs the question: why is she so afraid of you? Perhaps you are discussing his Will? Perhaps you are asking about his finances?
If you visit him, talk about the weather, baseball, the World Cup, fishing, classic cars. Assure his wife you are not after his estate. Put it in writing. Dig out his Will and show her she is the sole heir. Allay her fears.

Instead of fighting with your stepmother call the nursing home and speak to someone about this. Ask them if it's possible for your stepmother to deny you access to your dad. If it is possible, under what circumstances would the nursing home deny you access? What I mean is, can the nursing home just arbitrarily deny access because stepmother asks them to? Or would stepmother need some kind of legal document like a restraining order?

If your stepmother IS able to do this contact a lawyer immediately.

I take it that there is long-standing conflict between you and your father's wife. That is certainly unfortunate.

What is your father's health like? What are his impairments? Your profile lists general age-related decline, but as far as I know that is not something people go to a nursing home for. Does he have dementia? Is StepMother his health-care proxy?

Is there money involved in this situation? That is often a big factor in family conflicts.

I agree that she does have the right to keep you away.

You could take her to court if you have their money..

Case in point - Casey Kasem..

It sounds to me as if there are some basic courtesies to be observed that might help: telling her in advance when you plan to visit and asking if that is convenient for her, for example. Do you have any objection in principle to her being there when you visit your father?

Thanks to everyone for their helpful insights. Friction with stepmother is longstanding- she has been trying to eliminate my siblings and I from my father's life for years- gifts and cards from us are never seen again, while those from stepsibs are prominently on display as one small example. Things changed a bit for the better after we threatened to send Elder Services into the fray after our Dad had a fall, was unconscious for several minutes and step mother did not call for an ambulance and did not bring Dad to the doctor for TWO WEEKS. His health has been going downhill ever since. There were signs before that (Munchhausen's by proxy type signs), but two of us live out of state and in-state sib works 60 hours a week and is not able to be there as much as we would all like. Even before Dad was put in the nursing home, stepmother would look for ways to thwart our time together. The reason we would prefer to have visits without her is my Dad is not allowed to speak for himself- she insists on answering for him, talks over him or ignores him. Did I mention she is 25 years younger than Dad? Very impatient with him, cuts him off when he is speaking (even before health issues). Dad tends to just clam up when she is around. He admitted to me he is terrified of her withdrawing her 'support' and sticking him in a lousy facility. She talked Dad into signing everything over to her a couple of years ago. She has purged all of his personal belongings from the home they shared and relegated it to the garage which hurts him deeply. He does come home for major celebrations and so is aware of how she has removed his presence from the house. We just want to be able to spend what little time we have left with him without her steamrolling over our visits. She clearly feels threatened by this.