It sounds like you have a major headache on your hands!
Since it sounds like your mother lacks capacity to sign a new POA form, it sounds like guardianship is the last resort. You can find the Petition, here: http://courts.mi.gov/administration/scao/forms/courtforms/guardian-conservator/pc625.pdf
Oakland County Probate also has some helpful information on Guardianships, here: http://www.oakgov.com/courts/probate/Documents/in-house/1-guard-conserv-adults.pdf
Since your sister has the POA, you may not need to petition for conservatorship, as well. An attorney is not necessary, but you may find one helpful.
***Please be sure to mark if you find the answer "helpful" or a "best" answer. Thank you! I hope this helps. ***************************************** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state. I hope you our answer helpful!Ask a similar question
It sounds like someone would have to file for guardianship. However the Hospital might already have filed so check it out (if my memory serves they have to file if they keep someone involuntarily over 3 days).
However, your sister holding the POA may have preference to be appointed guardian. You will most likely need an attorney to do it right.
Please note that I answered this question with general knowlege of the law and with limited disclosure from you, my answer does not create an attorney-client relationship and I cannot be held responsible for how you, the asker, use the information provided. If you want to contact me directly and retain my services you may through my listing on this site or at 248-901-0750 or firstname.lastname@example.org.Ask a similar question