It's clear that your mom is being defensive, but what she does not realize is that she is in fact being defensive against herself, because it is an internal conflict in her mind.
For your part, of course you want to cut through the JW nonsense and reach her, but it's sort of like performing surgery on someone without anesthetics. Even though it may be necessary, it's going to be painful - and if someone doesn't know that they need to have the procedure then all they know is it will hurt, and they'll fight you tooth and nail.
It would be good to recognize that this is coming and prepare yourself, you can pretty much tell when you've touched on something that people are sensitive about - not to mention we all know those JW issues. The way I look at this is people basically are identified with certain ideas and beliefs, and if you challenge that they feel they themselves are threatened. Psychologically, it is easier to shift your identification to something else than face being obliterated. So I would recommend phrasing things in positive terms in a way that wouldn't obviously trigger her JW thinking, because then she will have some ways to identify other than the JW position.