Most people (sadly, even our parents or other people we respect) are not conditioned to update on a belief merely because it is true. Look at your mother's objections: she compared it to totalitarianism. If we take that objection at face value, then we know that she believes that such "narrow thinking" puts her at risk for totalitarianism, which is a risk she is not willilng to take for what is merely true.
Generally, if you want someone to believe something, you need to either trick them into believing that they already value what you are about to make them believe, or you need to trick them into modifying their values. This will be harder with your mother than with a stanger, as the idea has already been presented to her and she has parental authority to maintain. But there are ways to manipulate someone even in those circumstances, if that's what you wish to do.
I agree that generally raising someone's sanity waterline and getting them to think more rationally in everyday situations is a better approach to bringing them around to the naturalistic point of view than trying to force it on them.
From Being a Realist (even if you believe in God):
My mother, who doesn't call herself a theist (I think she's agnostic), doesn't even accept realism. She doesn't even agree with this:
That's little more than tautologies here. Yet it elicited an impression of being forced to believe. I know because she told me about the totalitarian dangers from such narrow thinking.
I'm happy to have finally found the root cause of our ongoing disagreement, but now, how can I deal with that? It looks pretty hopeless, but just in case, does someone have a suggestion, or should I just leave it at that? (My ego doesn't like it, but giving up is an option.)
Now I'm relieved to know that in near mode, she's a complete realist. This craziness only shows up in far mode.