What are POA rights?

Get legal advice from someone in your own jurisdiction. Guesses are dangerous.

We don't even have springing POAs in Az. It's general or Durable general POAs.

If your dad named his mom POA it does not go into effect until he is declared incompetent or he is unconscious and can not speak for himself.

He could ask her to handle business for him with the POA, but she can not decide to do anything on her own, unless the above is the case.

What is your dads status? That will help get you better answers. The more information that you provide the better the help. No personal information required, just info like dad is in a coma or he was preparing his documents because he has a terminal illness, he has a brain injury, you get what I mean.

This will also help you get information on how to help your dad get his property back.

Each state operates a little differently, so with the info you have provided I would say to talk to an attorney, they can help you understand how your state works and if there is any legal recourse. A lot of attorneys will give you a free consultation with the hopes that you will hire them to represent you and dad.

Grandfather gave son land. Grandmother felt as POA she had the right to turn it over to his sisters. Which, even if he isn't competent, she had no right to do if a will is in place. If he is confident, which he must be to assign her POA, then she was definitely wrong.

Was the POA drawn up by a lawyer, witnessed and notarized? If not, then she didn't have legal POA. And there r two. One for financial and one for medical. You need to find out what lawyer allowed this. Land changing hands has to be filed with the County Clerks office for tax purposes. Sounds fishy to me too.

Is dad competent? This is a confusing question. So land was deeded to your sister? Why did grandma want and think she could do this?

... your father gave his mother (your grandmother) POA ? ...and now your aunt is POA? (What....? Please clarify.) Sounds as if your dad wasn't at all aware of what he was signing, or was coerced somehow. Could he be evaluated from a doctor as to his level of competent ability now ... and when he signed/ gave her that authority it could be disputed - hopefully.
Sounds very fishy and manipulative on grandma's part.

If your Dad is in his right mind, Mom cannot do anything. He has to be where he can't make decisions for himself. Why would he make Mom POA?