MrsM We are/have been similar with ILs, DM, GM - they reject family advice and help with practical changes, yet are not capable of coping with eg GPs, admitting treatable illness, daily meds, reject support from social services. Comparing stories with friends, it's par for the course.
Solutions have been covert taking over of practical aspects of their lives rather than offering advice, in collusion with medics and any social service help you can get. Breezy uncompromising practicality has been a good approach. Get power of attorney otherwise you'll be funding them as well later on.
Huge sympathy. Am currently dealing with MIL not taking daily insulin replacement pill, loses mental function, leaves gas on etc, codependent with overbearing FIL obsessive about his major and minor ailments, complaining about her mental function but will not check her pills.
I know how you feel. Sometimes I leave them to it to pay the price of their pigheadedness. Then we have endure the unspoken criticism of other family and professionals with their 'useful' suggestions. Your mental health and your practical support network for their care come first, I would say.
At some point, you have to bulldoze: turn up at the GP without asking, talk over them, organise daily care, etc. Tell 'em how it's going to be for your own sanity and conscience.