My uncle is no longer able to care for himself and his two siblings are trying to figure out what to do next. Where do we start?

If the Uncle doesn't have dementia he can confer POA on one of the siblings; if things are in awful disarray that would be an onerous task. It may be better for them, if they are also of an age, to try to get him a court appointed guardian to handle all his affairs. They should see an Elder Law Attorney to plan out the possible moves forward.

Are siblings able to talk honestly to him? Ask him his wishes, whether he's made POA arrangements, etc.?  Is he willing/able to think clearly about his situation going forward? Perhaps they can explain what  his future might look like, with no one in the family knowing how to help him.