If it weren't for the trailer I never would have heard of the movie. Don't watch trailers? How many movie trailers deliberately mislead audiences on the plot of the film? This is one of the only examples I can think of. Not watching trailers is kind of bad advice.
That is not dead which can eternal lie, and with strange aeons even death may die.

Please refer back to the original post; the plot did NOT hinge on the revelation that Lukas was dead.
My advice is to stop watching trailers because they are meant as a marketing tool, not a synopsis of the movie. They either give too much away or, as in this case, misdirect the film to "get" a certain audience. They're useless. I may watch them after I've seen the film, but never before.

"In your opinion?"
"Um, yes your honor, in my opinion."

So get mad at the people who made the trailer.
How is it the filmmakers' fault that some American marketing firm f__ked up?

I see your point, but the trailer paints a completely different picture than what we got. When I saw the trailer it seemed really creepy. The mom may not be who she claims to be. That was what I was expecting. But in the first 5 minutes we can see that obviously Lukas isn't there/dead and it completely changes everything. For the rest of the movie I figured that couldn't be it, that the mother would be revealed to be an imposter. But it was't. Making me question the boys from the outset completely changes the dynamic of the film from what the trailer advertised to me.
And I agree a movie doesn't need a twist to be good. A great example of that being The Witch, a really creepy movie that was very straight forward. But it seems clear that this was hinging on the twist for it's plot. It just didn't pull it off very well. And I agree with you on characterization, tone etc. This movie does those things great. But the horror aspect gets lost in this horror film when you can figure it out from the start. Plot goes a long way sometimes and acting and tone have to be put aside.
I really wanted to like this movie, but it's plot was just so obvious it was kind of boring.
That is not dead which can eternal lie, and with strange aeons even death may die.

Couldn't say it better myself. My thoughts exactly

Well said.
Any horror fan would have guessed in the first few minutes when mother only pours drink for one boy
So, many knew what the twist might be but looked forward to its delivery.

It wasn't a twist. It was obvious almost from the get go that there were no twins.

Well said

Totally agree. The "twist" was the full explanation for one aspect of the story, something we knew was a bit "off" but it was NOT the point of the movie. It was one small piece in a film about much bigger and more intriguing themes. It wasn't even really a "twist", but a reveal, which pretty much every story has in some form or another.
There's not much else to say as the poster above did a good job.