He has strayed far from God and unlikely to return

Probably tempting to cut off your sister and him, but I'd offer another path that will make him much more miserable. If they have or end up having kids... slowly radicalize them. You can probably start young but gotta be careful because kids will just say shit to their parents. But once they're in high school there's a lot of opportunity to just expose them to all sorts of shit that their dad will hate them learning. Probably the easiest thing to do though is be active in politics, belonging to organizations that do things beyond like reading clubs (which are good too), and have your future niece/nephew maybe go along, see stuff, be the cool counter to their asshole dad, and yeah.

Not much else to be done besides... well, I shouldn't say that. Anyway. Oh, by the way!...