My 88 year old mother has Alzheimer's. Her sister, who she does not see on a regular basis, is near death. Should I tell her?

I feel like she cannot process grief, that she will have to hear it over and over and it will be devastating each time. This is her only sibling, they were close over the years, but not in close contact for the last 10 years. I am concerned on how it will affect her health (both mentally and physically). I am considering not telling her when her sister dies. She would have found this appalling before her illness. Now, I feel like she can only live in the moment and even if we have a week of telling her over and over and drag her through a funeral, that she will not remember it for long. So what is the point? She has been robbed of so much, doesn't she deserve to be spared what she is not capable of handling?