It sounds awful.
One silver lining for me has been the realisation that I am far from the only one with a difficult relationship with my mother. I spent far too many years accepting that my brother was the golden boy because he was in some way "better". And that everyone else had close nurturing families, a bit like on TV.
Having a difficult relationship with your mother is perhaps something that not many people talk about, expect now when that relationship is reversed and the daughter needs to step up to the plate. Not least because the golden boy brother does not seem to be able to cope, again something that appears to be fairly standard.
On the longer thread I shared my cousin's view that many in our mother's generation are frustrated and angry. They had the intelligence but even if they had the education they were not allowed to use it. Husbands and family came first.
I could have found plenty of reasons to duck out, and the transition where I had to insist I took control was pretty awful. But I am glad I did. As you will know from your brother there are times when you know what is the right approach even though emotionally your mother is reluctant.
I would be guided by the health and SS professionals. If her tests show she needs support and, say, is financially vulnerable, then use that to do your best to insist she hands over the necessary control and that you get the support you need. If she "passes" the tests and is being obstinate, then there is probably not much you can do. (Her symptoms could be caused by something as simple as a UTI.) Save your energy for later. And do if you can, seek advice from the same professionals on your own situation. Some, though not all, were both experienced and wise and helped me get through the transition to getting my mother into an environment where she has as much independence as she can manage whilst being protected.
Having had a husband die of dementia and having been a carer for her son she must be pretty frightened at what the future holds. It cant be easy for you either, knowing what may be coming up.