How can a will be created for my father if he suffers from dementia?

My dad suffers from prominent bouts of dementia due to being in the advanced stages of Parkinson's Disease. Years ago we went to an attorney to give me POA. Over the years, I have had to use this many times for various issues.

According to him, he has a will. But I have gone over every scrap of paperwork I have come across and cannot find one. Nor had he ever mentioned an attorney who drew up this will. In his stage now, how can a will be done?

There are 4 of us siblings and my dad has discussed with all of us (years ago) his wishes & basically anything left is to be split 4 ways. Being that I am the cargiver for him, I'm feeling uncomfortable about him not having a will. My other siblings don't question how I care for our dad or how his money is spent for his care, but I've moved into his home and care for him there (I couldn't hold down 2 households) & I just don't want to have any painful issues after he passes that I hear & have seen so many horror stories after a parent's passing.

Any help, thoughts or ideas will be greatly appreciated.