My father with dementia’s brother just passed away. We have not been notified of his estate. Is there anything I should do on his behalf?

Just relax. Asking about the will will only make you appear greedy. It is way too soon to even think about settling the estate of notifying beneficiaries. Dad may very well not inherit a dime.

A few days after death is kind of jumping the gun in relation to estate issues. Let the dust settle for a few weeks before getting worked up about the will.

I don't see why you couldn't ask the person who WAS (and is no longer) POA if they know anything about the uncle's estate, but again, wait a bit. If Uncle had a trust (and with a lot of assets, he should have) as well as a will, but the will might not say much of anything if everything's in the trust. My parents' wills basically said, "I leave all my stuff to my spouse," and that was it, because the house and all the bank accounts were in the trust.

I didn't even bother filing my mother's will, because it cost $50 to file my dad's and it didn't say anything. I wasn't going to waste another $50 just so the county could keep a copy on file. No one's ever going to look it up, and if they did it wouldn't have told them anything anyway.

A POA has no power to sell the home of a deceased's estate. The POA ends with death. However, if someone was named POA by your Dad's brother, then that person was likely ALSO named as the Executor of the Estate (if there is a will) or the Trustee of Trust (if there was a Trust. Both a Trustee and an Executor must notify you within a given time frame (look up the laws of your state concerned) IF you are a beneficiary.
Yes, of course you should reach out to this person who WAS the POA, and you should ask if they are also the Executor of Estate or the Trustee of any Trust. There may, after all have been NO will or Estate left. You should feel free to tell them that any funds that may have been left by this gentleman to his brother would as you told us, greatly impact his life for the better. You are doing nothing wrong.
Meanwhile it hurts nothing to check with the records office to see if Probate has been opened (in the case of a will; trusts aren't filed) as this must be legally recorded. If the will IS legally recorded you are able to read it.
If you meet roadblocks that you feel indicate someone is hiding something important, then feel free to contact a Trust and Estate Attorney who can write letters of concerned next of kin or their Fiduciary (ARE you POA for your Dad) asking for notification if there are any family members who are bequeathed any funds or property.
I sure wish you the best and I hope you'll update us. It sounds as though these cousins (progeny of another sister?) were most in touch with and caring for your Dad's bro before his passing.
Do know that legally the Executor or Administrator or Trustee of an Estate have time for notify beneficiaries; as I said the time is dictated by state law.

When someone dies, its not like it is on TV. After the funeral everyone is called into the study and the Will is read by the lawyer and beneficiaries told what they inherit. There is so much more that goes into Probating an estate.

The next of kin thing only goes into effect when there is no Will and there is an estate.

The house must have been sold prior to his death. Maybe he was in care and the money was needed to pay for it. Or just put in his account.

Probate can't even be done till a certain number of days after death. My State 9 or 10 days. The POA has no power after death. The estate becomes the responsibility of the Executor. The Executor goes to probate and has the Will put on file. The Will is now public and anyone can request a copy. Its the Executors responsibility to notify the beneficiaries and interested parties that Will is now in Probate. A copy of the Will is sent along with the notification or available upon request. In my State probate cannot be closed for 8 months. Creditors have to be given time to get any claims against the estate filed. And the Executor has a lot to deal with. You need tax numbers from the government. Have to make sure all bills and liens are paid. Its not just cut and dry. An accting is done and each beneficiary has to sign off on it. If its contested, then it will take that much longer to close out Probate.

If Uncle died without a Will, then the State determines who inherits. But if Uncle had a POA he probably had a Will. Yes, there is no law that Uncle needs to leave his money to siblings. Seems like you and Dad were not close to this Uncle if it took a cousin 2 days to contact you. I would not count on ur Dad getting anything. Be surprised when he does.

A person can will all their fortune to a cat if they so desire. There is nothing 'illegal' about not mentioning your father in his twin-brother's will, IF that is the case. If he had a will, then your dad will be notified if he was mentioned in it. If not, you have no legal recourse here, nor do you have the right to say something to the POA/executor of the will besides offering your condolences on your uncle's passing.

Probate will notify your father if he was mentioned in the will. Your uncle was under no obligation to leave his brother anything. If he died without a will, probate will determine who inherits.

Since your cousin was his POA I'm going to assume he also had his will properly prepared, and while an inheritance would be a nice little bonus the man had no obligation to leave you or your father anything. In any case it can take up to a year to settle a complicated estate, rest assured you will be informed in due time if named as a beneficiary. Seriously, if the man only died a few days ago they've hardly had time to deal with his funeral let alone his estate.