You may have to look into a POA (power of attorney) so that you can make legal and health decisions on her behalf.
Quite honestly, anyone who has a loved one with cancer or any other life threatening illness should have a POA in place. I am so glad my mom had one when she was diagnosed since I was then able to access medical tests and records, financial documents and accounts, etc.
Luckily, my mom put her POA in place herself for me. You will have to consult and attorney to get a POA and have mom declared "incompetent"...yes, horrible word, but if she needs care and Hospice should be there for her you may well have to be proactive in order to get her the care she needs.
It's very hard to deal with some of these issues and it's made more difficult by the fact that many patients suffer mental impairment toward the end of their disease process due to either brain mets or toxins form kidney or liver failure.
It is so critical to have that POA before a patient gets to that point or you will be left unable to help them.
Take care of yourself!
Jen