Rights as and obtaining power of attorny

My grandmother, who is a LONG time heavy alcoholic and has a history of delusions wound up getting herself locked up in the psych ward of the hospital last week. I agree that she's not all there and she does need some serious intervention, however, she doesn't belong where she is... what's worse, she wants out. She wants out so badly that she is considering giving my great uncle power of attorney over her in hopes that he can get her out. My great uncle is not a good man (READ: very intelligent and VERY devious) who I know will not look out for her best interest. I am afraid that he would gladly seize control of her assets and keep her right where she is. Unfortunately he looks beautiful on paper. (war veteran, history professor, etc, etc.) My mother is next of kin but she has a past. She is a recovered drug addict and was a prostitute for 14 years and has several felonies on her permanent record. She has been clean for three years, working, going to college but compared to my Uncle doesn't have a leg to stand on, so unfortunately if it came to fighting him it would fall on me.

I need to know two things if anyone has this type of knowledge. First of all, I need to know if she granted ANYONE power of attorney, would this give that person the right and the leverage to get her out of the hospital? Secondly, I need to know that if she is dead set on granting this power to her brother, my great uncle is there a way that I can argue that though she's not crazy enough to be kept where she is, she is not in the right frame of mind to make this decision so that I can fight my uncle if I have to?