This is so sad! Astonishing how this religion can make perfectly nice people do evil things.
Is there any way you could have your Mom live with you? Even part time, like 6 months out of the year? No one has the right to keep parents and children apart, esp. when the parent is older. Was it the Awake or WT that had an article a while back that said something about religion should not break families up? If you could get a copy of that, perhaps you could show it to her, and ask why she is not following the wt advice? You sister would probably monitor any cell phone you gave your Mom.
And please make sure no one knows about your nephew asking your daughter questions! Perhaps he can be saved from this lousy group.
All the best to you.