Does he have money?

I agree that you are well within your rights to walk away, and I also understand the guilt you are experiencing.

I'd suggest a middle way for now - take a break and maybe offer help again in a couple of weeks. At some point he's going to realize he needs it and will have to act decent to get it, or he won't and then you can really fully let go guilt free.

You can walk away. You can be straightforward that you are estranged, and do not wish to participate in his care. You are not his POA and are not legally obligated. The facility will have to find a safe discharge for him. If he has sound mind, he will make his own decisions. If he loses capacity, it’s up to you whether you want to make decisions on his behalf, without any contact, but with the history you describe - you are 100% within your rights to wash your hands of him. If your sister remains involved, and is resentful of your walking away, you can remind her that she has the same choice.

You should have cut contact 40 years ago.

Be done! And get a therapist.