Well, what was your grandfather preaching about in particular? Perhaps your mother doesn't agree with his interpretation, or as others have mentioned, resents being preached to in her own home when she doesn't fully accept the doctrine.
I also noticed that you mentioned he was your grandfather on your
father's side. To me, this coupled with the statement that "she's either a non-believer or has no idea what Bible things mean" suggest that she may not have been very religious before marrying your father. Maybe she resents her father-in-law telling her what to believe. Going to church, even if not a believer herself, may be a concession of sorts to the family she married into.
I can't say anything for certain without more information, but that's what first comes to mind.
D i v i n i t y
Try praying for her and talking to her going "don't you believe that there's a God? Someone must have created all of us. The Bible can help you believe or help answer some questions. Give it a chance and try reading a bit of it."
See what she says then?
As well intentioned as this response is, I doubt it'd be well-received. The phrasing is just a little condecending, suggesting that it seems absurd she can't comprehend something so simple (although I'm sure you had no intention of implying that). Asking her what she believes personally as Shiroi suggested would be a better approach. As of now, you don't seem to know much at all about what your mother really believes, and you may find the conversation enlightening.