So sorry to hear about this.
It seems like it would be very difficult for her to change now, given her age. If she doesn't want to change, there's not a whole lot one can do. You can offer her some help from Psychologists/Psychiatrists. Getting to the root of the problem I think is the key. You need to find out why she hoards. Is she depressed/lonely, or does she just not like to throw things away, or is she just bad at organizing things, or all of the above?
My mom is a hoarder too, mostly of religious items though, and she is very bad at organizing things, and spends almost no time at cleaning up after herself. If she ever did learn to organize and clean-up after herself, I think she would be fine for the most part.
I'll keep you in my prayers! God bless!