I'm my father's POA. He's 86 and my brother moved in with him a few years ago because he had nowhere to go. He works and then comes home and cooks my father's meals, but doesn't pay attention to his health. Also, always wants money from him. I want to move in to take proper care of my father but what can I do about my brother?
That's the only question that matters here.
If your father is diagnosed and judged incompetent in his own care by however MDs your POA includes, then you are in charge of him and have a Fiduciary duty to protect him and his money. This means you are in charge of all accounts as POA, and can make him a small allowance of his own to share with whomever he likes, but his money is in his name with YOU as POA paying all bills. If you don't understand how to do POA check signing and account set up you have to go to either an elder law attorney or a Bank OFFICER to find out and to get things set up.
If your father is incompetent you now must act for him. You can do any and all things that your POA SAYS you can do. And none that it doesn't say you can do.
You should be able to evict your brother. At that point you can move in.
Now if your father IS competent to make his own decisions then it is his decision who lives with him and it is his decision who he gives any and all money to. If your father is not diagnosed with severe dementia and incompetence in his own safe care then he can even CHANGE his POA from you to the brother if he wishes to.
You are POA. I strongly advise you to see an elder law attorney (this is paid for by your father's funds) to find out your options, how to operate as POA, what powers you have, and how to do meticulous record keeping. Take your POA document with you to the attorney who drew it up if available, or to an elder law attorney. And good luck.
Why should the father have to spend his money on an elder attorney to teach the OP how to be a POA?
Most POA documents are pretty self-explanatory. As for the record keeping, that's pretty easy too. You save all the receipts and keep a ledger. I was the POA for three different people, two at the same time and this is how to do it.
The probate court in the town the father lives in can also be a valuable resource to a POA. They will also let a person know how much they can legally charge for their POA services. No one has to do it for free and many people don't know that.
If your POA is active because your father is incapacitated and mentally incompetent, and it covers more than just your father's financial matters, you would be the one responsible for making sure that he is safe and his basic needs (food, shelter, medical care) are being met. This is not your brother's responsibility. It's yours. In such case, you can transfer all of your father's bank accounts so he can't access money. Then you can legally evict your brother from the house.
If your father is not incapacitated or mentally incompetent, then he can have whoever he wants living in his house. You would have no right to kick your brother out if your father wants him there to then move yourself in.
I would suggest you have a meeting with your father and brother.