Oy, sorry to hear about your troubles, man. I assume your mother is in her 60's?
Well, for some older people, especially when they lose their spouses, they want something new to cling to, because they are so distraught over their spouses' death. I know how the witnesses prey on people who are depressed, traumatized from a loved one's death, etc. Yet, if you can curb your mother's tirades, perhaps just tell her unequivocally that you want nothing to do with her religion, but that you love her just the same, and whatever makes her happy, you're happy with, too.
I'm afraid that with new converts, there's really no way to pull them out. You mother's probably reach many of her life's milestones, and now she wants new goals, and that's something that the witnesses give.
Now, I'm only 22, but I grew up as a witness, so I know what the deal is. I noticed that the older people who were zealous were paraded around the hall as 'examples', and your mother may be one of them. They may laud her ability to preach about the bible.
What the JWs are about is the very clever stroking of the ego, which in turn controls the person. Your mother may be perfectly happy as a JW. Just don't let your child be exposed to their insidious talk. Remember, she's your mother, not your gatekeeper.
I would recommend to just find out more about her hall where she meets, but tell her you have no interest in the religion. This way, you can find out what nights she goes to church, and don't let your child sleep-over on those nights...she may bring her and try to subtly indoctrinate her, although the witnesses would deny trying to so to to kids.
Remember, these people 'love-bomb' all newcomers, but many old people find friends at this church, and end up staying because it's an ok faith for people who don't need to have any moral conviction. This is a religion that thinks for them.
Anyway, feel free to email me if you need to....it's open.
And don't start fights with her about it!!!!! The VERY FIRST THING WITNESSES TELL BIBLE STUDIES IS THAT SATAN WILL USE FAMILY TO TRY TO DISSUADE THEM FROM 'STUDYING THE BIBLE' (which is Witness-Speak for becoming a Jehovah's Witness)
Take it easy buddy.