I’m going to borrow an illustration from Buddhism:
Let’s say there is a man named Jack who has a phobia about snakes. Jack walks into a dimly lit room, sees a snake coiled up in the corner, and panics. In fact, he is looking at a striped necktie, but in his terror he has misinterpreted what he sees to the point that he could even die of fright—death caused by a snake that does not truly exist.
Fortunately for Jack, his friend Jill walks into the room. Jill is calm and sane and knows that Jack thinks he sees a snake. She can switch on the light and reveal that there is no snake.
When Jill finds Jack frozen in terror, she has some choices about what to do. She can directly point out that there is no snake or she can use a skillful method such as escorting the “snake” from the room. But if Jack is so terrified that he is unable to differentiate the snake from the tie, even with the lights on, and if Jill is not skillful, then she can actually make things worse. If she dangles the necktie in front of Jack’s face, he could die of a heart attack.
But if Jill is skillful and sees that Jack is delusional, she can say, “Yes, I see the snake,” and carefully take the tie out of the room so that Jack feels safe for the time being. Maybe then, when he is relaxed, he can be gently guided to the point of seeing that there was never a snake in the first place.—What Makes You Not a Buddhist by Dzongsar Jamyang Khyentse
I think you can see where I’m going with this. Drop an apostate bombshell on your mom and she may not be able to deal with it. If you can start from a common perspective you may be able to gently guide her to see the truth rather than alienate her.
Do you have a back-up place to live in case it all goes to hell?