Seriously, if any of you "very serious people" want some advice on what you can do to stop the fascists, I think you entire approach on how you talk to them needs to change.
You're starting from the idea that these people are dupes, primarily because you are NOT a fascist, so you don't really understand how people can KNOWINGLY choose fascism - thus you assume grandpa has been tricked.
This is not so, no matter how much they lie to you about it.