What should I do if I have an older aunt with no children?

Need more information.  Does the aunt live with someone else?  Even if the aunt owns the house, I can see someone saying visiting restricted to say living room and aunt's room.  It is very hard to say without knowing more, is this someone trying to isolate your aunt, or just manage the situation.  

My mom lives with me.   I love it when siblings come to see her, but I do like them to let me know ahead of time.  My siblings know better than to snoop through my study or my room, but if they didn't I would say something.

Can you tell us a bit more information please. Not sure it is clear. Your profile says that you are caring for a 98 year old. Is that the aunt? Yet, here you say someone is living alone. Sorry, I am confused. Please clarify for me. Thanks.

I am semi in this position. However in my case I am the niece in law. My husband is one of the original 13 nieces and nephews. The aunt, 93 and with dementia, has no children of her own. Several of her nephews have passed. Only one niece is in the area and her son and his wife actually help me with Aunts care. I have known Aunt for 40 yrs. I’ve been her POA for 13 yrs. I can tell you there have been times when I would like to put the restrictions on relatives that you mention but would feel those actions are out of my authority unless Aunt requested it or I thought the relatives were upsetting to her. It’s more upsetting to me than her so I let it go.
But to answer your question. Is your aunt competent? If so she can appoint anyone she chooses as her POA. You could file for guardianship and might win but might not and then be in worse shape. You could also ask aunt to appoint you and or your siblings as her POA. My Aunt has only ever agreed to have one other nephew to be her POA and that was at my insistence and he was secondary to me. He died and she replaced him as secondary with my husband. I wanted a blood relation involved and I wanted a secondary but that was just my request. It’s not required.
Believe me, no one wanted the actual job.

No POA document I’ve ever seen could prohibit you from visiting or speaking freely with aunt but there are many posts on this site where POAs have prohibited siblings from seeing their parents or even knowing where they were. There is that old saying that “possession is nine-tenths the law”. Not saying your aunt is property but if you look this up, you’ll see it feels true.
What you need is a certified elder attorney to advise you and an agreeable aunt. If her nieces and nephews talked up NH placement etc or other actions that were frightening to her in the past, they (you) might have scared her into this action. What happened to your parents or other people she knew intimately may have been a cautionary tale to her. I assume none of her siblings are living? What about church or friends or neighbors that might fill in the blanks on how this person gained control of aunts POA?
I would visit often and take photos to satisfy yourself that she is doing well.
I can walk into my aunts home and within a few minutes know what’s going on and how she is doing.
It would bother me if my aunt did what yours has done but it is her right.
If you don’t think she’s competent or that the POA is honorable then don’t delay taking action.
Many attorneys will give a free initial consultation.