In your opinion, what is it that distinguishes your grandma from people in online discussions? I've been thinking about it a lot lately - that the facelessness of it makes it more difficult to employ empathy? That there are too many people online and connections with them are too ephemeral to form emotions necessary for kind discussions? That the anonymity makes some people lose inhibitions? What else?
I suppose you have to get along with grandma. You have a shared family group. Imaging taking to your mom, and saying "I've cancelled grandma because she thinks that Napoleon was no better than Genghis Khan, so no more family meet ups. If you don't cut ties with her, you're blocked and cancelled too."
There's also the fact that you've known grandma your whole life, and you think that she's 90% adorable, 10% a product of her generation. So you're willing to change the subject, or maybe just roll your eyes, when it comes Napoleon.