I'm afraid I've become a caregiver by default.

I agree with the preceeding, however, possibly mentioning to the daughter that you really want to remain her mother's friend and that caregiving beyond..... (whatever you choose to take on)is something you feel will put a strain on that relationship. The doctor should be able to order nursing care for her, there are buses in most cities for the handicapped and elderly and an ad in the local paper might find a willing mom, college or older high school student willing to run errands for her.