You ask your question under the "cancer" forum but you don't mention that in your profile. At 89 yrs old, however, she may have undiagnosed dementia. If she has dementia, the symptoms of not knowing you or where she is, is certainly a common symptom, one that can be heartbreaking for the caregiver. Introduce yourself to her and tell her you are taking care of her. If it's not dementia related, let the hospice nurse know what's going on and get her advice.
It is also common when they are shutting down.
I am so sorry for your loss.