How does Dad feel about you getting him some respite care? No one should have to deal with this 24/7. It is quite common for the mentally ill person wearing down the care-partner and killing (for lack of a better term) them off long before the dementing disease gets the patient.
Have you folks tried adult day care?. There are programs for those with dementia which can be stimulating.
IME, parents don't wish to burden their children with their problems and this habit seems to be very ingrained. How you handle dad's revelations will likely dictate how forthcoming he is. Remember, the world he is living in is very different than ours; it is the world created by dealing with and living with the mentally ill. If you were to send dad on a cruise and take care of mom for a week 24/7, you'd know (if you don't already) exactly what I mean.
I think your instinct about talking to your sibling and formulating a plan of action is a good one. You should consider getting a POA for dad's health and financial care if something were to happen to him, as mom isn't competent to assume those responsibilities. As he is managing their assets currently, you could assume those responsibilities, and, tangentially, those of mom's care, if he were to unexpectedly fall ill or become demented as well.
Best wishes!
Pat