Am same condition/disease process as your sister. Is she on any steroid drugs such as Decadron? Side effects include increased irritability and volatility....as I can attest.
Am a retired Registered Nurse. Once upon a year, I was having difficulty with my father and spoke with a co worker psychiatrist who said. "Gail, old age does not convey sainthood. If someone is a miserable adult, they will be a miserable oldster." I suggest that serious illness has not conveyed any sainthood on your sister....but, perhaps, a really handy excuse for misbehaving.
Suggestions 1) In her presence, speak to her attending about her behavior and see if change in anti depressant drugs would be helpful. 2) Speak with the other family caregivers and establish limits for the patient. Tell the patient sister....and have consequences. If you are ugly, we will still provide essentials but, will leave as soon as completed?