Did I somehow miss it or did they...[major spoilers]

I'm thinking it wasn't blatant (even though it really was lol) so as to not spoon feed it to the audience.

O wow... I didn't know that about them... It makes sense now why she killed their child.

if they mentioned it, I didn't see it or pick up on it and I thought the movie was decent so I was paying attention. makes sense now.

I wasn't paying much attention to the movie as I played in the background doing other stuff.

I totally didn't get the incest thing... had to learn about it through IMDB reviewers who had read the book.

Having that revealed would have made the viewing at least more interesting but there's still too many things that don't make sense. Why for instance, did Vernon go on a killing spree? He just killed people he encountered who didn't seem to have anything to do with his mission. That's lazy story telling and a trail of innocents dead in his wake does not make him a character the casual viewer can understand. The killing was overkill.

The whole movie was bizarre and unsettling with no real conclusion.

I just didn't get it. No idea they were brother and sister. And I paid attention.

I had no idea about that. Full disclosure: the movie got very boring (even when Cheeon was shooting his gun for 20 minutes) so I started paying less and less attention.

Thank you for clarifying this. I started to wonder early on and I mentioned it to my partner that I thought that might be it, but neither one of us caught any verification in the film. I'm assuming you've read the book, did the Vern rape the Medora? I was just wondering because of the hand around the throat that happened in both the bed with Russel and again in the hot spring with Vern.