Sorry to hear about your Father, it's quite a blow when they walk into the hospital voluntarily and never walk out. The same thing happened to my father and on top of that it was a routine surgery but it turned out he needed blood a day or so later, due to a leak in an artery. He was too weak to un-sign the blood directive and didn't survive. On top of that, he wasn't even a JW....just studied with JW's. A short time later, they OK'd the use of "blood fractions" which would have saved him.
I was curious if your DF'd niece was told that her great grandmother wanted to get in touch with her ? If not, perhaps you could get her to make the call and somehow make sure Grandma is available to answer the phone directly so as to avoid interference by your Aunt.
It seems to me that old folks have on some level realized what's truly important and because of their limited time left, are willing to cut through all the nonsense.