Is there anything I can do to contest my sister's right of having a Power of Attorney over my father who suffers from dementia and Alzheimer's?

Yes, Coach is right with getting an attorney. Proving one is not mentally capable can be a touchy thing. That requires documentation from all doctors involved. Deeming someone incompentent is a court battle. If you decide to go down this road remember that the court system can rule against you or your sister becoming guardian. They can likely appoint an outside third party that would better suit as guardian to make the decisons in the best interest of your father. Than neither you or your sister will be able to make decisions for him.
I had some problems here with my mother and siblings. I talked to her drs and talked to an attorney. I felt that persuing the matter and risk losing mom to an court appointed guardian all because us kids was fighting with each other wouldn't be fair to our mother.
The best thing is to prove that your sister is not doing her duties of POA. Not helping out has really no grounds. But proof that she is financially using his money for herself, things along them lines. That will get POA overturned. Again, you will go to court.
But if in fact your sister is doing her POA duties, and this is about life support issues than she has the say so unless there was a living will.

Again, talk to an attorney ASAP. These things don't change overnight. So the sooner you seek legal advice the better.
As coach said" he (your father) is of primary concern and should be properly cared for".

Yes there is but you will have to get an attorney and use the court system. You will have to prove to the court that your father had Alzheimer's/Dementia at the time your sister was "appointed" as POA and that he was incapable of making such a decision making the POA invalid. If your father prepared a Living Will prior to being incapacitated, your argument that he never wanted to kept alive by artificial means would be much easier to prove. If he does not have a Living Will it will be your word against the word of your sister.

Be prepared for this to get nasty should you proceed. Before you do anything try talking to your sister in a civil manner. Express your wish to be the POA as you are the primary caregiver and express your father's wish not to be kept alive by artificial means. Whatever happens between you and your sibling, make sure your father is properly cared for. He is of primary concern as he can no long express his needs and desires. Good Luck.

There is no such thing as a right to have a POA. Granting such [power is purely a voluntary act of the grantor. Unless the POA is what is known as a Durable Power of Attorney, by your father's incompetency, it is commonly held to be no longer valid. And BTW it is also voided by death. This is a problem too complex for you to handle without a lawyer. If finances are a problem, your father's assets are available for that expense.

POA is given to a person or (people) for a reason, the reason is they cannot handle thier own affairs. Being diagnosed with Alz/Dementia does not equal incompentancy. So any papers signed "pror" to any legally document saying that a person is incompitent in decision making, is going to be a difficult fact to prove otherwise. In court proof is all that matters. Proof of abusing powers is another way to go. Those papers state the matters that an acting agent (POA) should be acting on. Read the POA papers (if you can) see if it gives the Agent that power to DNA. Living Will gives the Power of health end of life issues too. The hospital your Dad is in should have DNA papers, if your father can still sign them, he can give you that power that way, talk to Social Worker at the hospital.
Most families go through this conflict because there are different personalities & if everyone could get along and agree on things, Lawyers for the most part wouldn't be needed. With that said, explain to your sister your concerns and try to reach an agreement in a peaceful way. The bottom line is your parents well being and "thier" wishes are priority. Forget about who does what and who dosen't. If this dosen't work or is an impossability, I would say you leave me no choice it is going to cost a fortune to fight it out but be prepared for your bank account to be emptied over this matter. My sisters first concern is her $$$, it took me a while to figure out how to get my sister to agree, but she listens up when I mention $$$$...you do it my way or you'll pay $$$. She agrees every time. She was mad because I spent down Mom's money got her on medicaid and Mom set until the end (and sis found a problem with this). Sis realized no left overs for her. I said OK I'll tell medicaid you'll pay $$10,000 a month out of your own pocket for NH or I'll take Mom out of NH and bring her to your house and you figure out a better way to care for her for 24/7 for years to come. Well she never said another word about it to me. This is what I ment about personalities you've got to figure out what motivates a person, loss of freedom, money, whatever gets them to wake up and get the real picture and who's lives and wishes are the priority. Fighting and disagreeing usually makes someone else alot of money and drains everyones bank accounts and energy. Oh and some people are better readers than listeners so a letter goes much further than a phone call sometimes.
You are a family... try to use what your parents gave you life and individuality for, as motive to help them.

I think you need to apply for guardianship, since you are the caregiver, you should get it, not sure thou. Do call a lawyer, its free the first time you go for a consult. Best of Luck

My wife's father recently passed of Alzheimer's at age 74. Her mother passed 4 years prior of Parkinson's and her sister just recently died two years ago, she is the only one left. After her mother died her father met a women on Match site or something and married her within 1 and half years. My wife has always been close to her parents and had a good relationship. She could clearly see what was happening to her father with this new women. Soon this women had POA and then was re-writing the will removing my wife from it and put the house in her name. My father in laws best friend told us we need to fight this is court because he knows she is in it only for his money, nothing else. We don't even know what he is worth because who talks to their father when he is dying of a disease that impacts his thinking and doesn't make good sense as it is. We saw this when my wife said don't marry her and he said he must and married her on his ex-wife's anniversary. It was just more than my wife could take. Consequently we have access to nothing paper work belongings, nothing and this lady who we thought was somewhat normal at one time has showing her true colors and made it obvious that she is there for the money. We assume my wife's father had some money because this lady told my father in-law 's best friend that she will be sitting good now. Also she started selling his stuff a few moths ago before he even passed. What can my wife do?

Laws vary from state to state. For example in my state (Oregon) there is a distinction between a Guardian and a Conservator. You urgently need legal advice. The problem is not overly complex but you cannot proceed without a lawyer. BTW I am retired and not available.

I am sure the doctors will talk to your sister about life support when the time comes to make that decision my husband's doctor talked with all of us -me my son and grand-daughter and we had paperwork that stated what his wishes were and at the time his brain and heart had stopped functioning. You could try talking to her but it seems she is not considering how you feel at all and it might be more upseting to you to talk to her about it again. I am so sorry for what you are going through and I hope you have someone close to you to give you support at this time.

Depending upon your father's current condition, you may or may not have time to attempt to obtain a guardianship or conservatorship - the process generally takes up to 6 months or even longer. You mentioned that he is in ICU but I wasn't quite sure if he was on life support or not.
You can challenge the POA (is this a durable power of attorney for health care and does it include a durable power of attorney for finances, as well) by use of an attorney IF there was a written diagnosis of Alzheimer's disease or other dementia prior to the appointment of the POA. Either way, call your local area agency on aging and ask to speak to their elderlaw attorney. They can usually set you up with a free telephone consult. Make sure that ANY attorney you consult has a specialty in elder law.
You may want to consider having the social worker, an outside geriatric care manager or a family mediator sit down with your family and discuss the alternatives.
I'm so sorry that you're going through this and I pray for the best for your father.

Shelley

Determining a persons competency can typically be done by asking them to answer a few simple questions to show they are oriented to time (ask the time), place (ask the place they are now), and person (ask them who they are). Some add purpose if they are in the hospital (why are you here?). If they can easily answer they questions, a physican or any other person would be hard pressed to determine in their professional opinion that that person was not oriented and thus not able to continue to make decisions about their own care. It doesnt mean they are not limited in their mobility. Many people that work every day are limited in their mobility. Yes, you can contest actions but also realize that some persons have recovered when no one thought that they would and to terminate their lives by pulling a plug might be a decision that could haunt you into the future. Clearly someone unconscious is the a good example for POA or HCPOA etc. but sitting down with the POA and having some dialogue may be the best course of action.