Mom is ninety. She had an ischemic stroke two years ago. She has had, at her request, untreated breast cancer. Any thoughts?

I am sorry that you are losing your mom.

My sister died of untreated breast cancer and before she died it had metastasized to her bones and her brain, it got very funky before it was over. So be mindful that she could be experiencing something similar.

Sometimes we just have to say no. It is okay to keep boundaries with a dying person.

I believe that in all likelihood what BarbinBrooklyn has said is dead on. Your Mom's stroke has likely caused some vascular dementia. If this is the case, "tough love" would be somewhat of a joke. People with dementia are completely unable to act and react within the rules and norms, as they are not a part of how their brains function anymore. I am sorry for this Stage 4 diagnosis, but with untreated breast cancer you could also well be looking at a spread of cancer to the brain. Breast cancer most often metastasizes to bones, lungs and brain. This would also cause a mind that is unable to react in the normal manner.
I do not know what medical followup you are getting for your Mom? I hope her papers are all in order already for her to be represented by you in her behalf when she is unable to do so. If you have some scans it may well tell you when it is close to the time for you to call in hospice care to assist both you and Mom with this last stage of her life. I am so sorry for all you are both going through. It has to be so very difficult for you.

Sometimes stroke brings on vascular dementia, which results in what looks like mental issues.

Has she been assessed by a geriatric psychiatrist? That would be the best course of action.