So what were your twist predictions? (SPOILERS, obviously)
A lot of people say they suspected from the beginning that the "grandparents" weren't really the kids' grandparents. I personally didn't see that coming until pretty close to its actual reveal. Because of the grandma's vomiting, I thought it was going to be some kind of demon-possession thing, because it reminded me of The Exorcist. I almost think that might have been a red herring, because that was some seriously projectile vomit. And the way she crawled under the house - it looked super-human. It was way too fast and agile for an elderly woman. She looked like a crab or something. So that also made me think there was something supernatural going on.
I also theorized that the grandma was just an evil person doing some kind of experimenting on the grandpa (because of their work at the hospital counseling, maybe she wanted to make a name for herself as a psychologist), and that's what made him act strange and poop himself, whereas she was just plain evil.
I also thought maybe they really weren't evil at all, and it was going to be a story about not letting your imagination get the best of you or something. Maybe they would have killed the grandparents in self-defense, only they didn't actually need to.
Also, with the mom having done something to the parents she wouldn't 'fess up to, I thought maybe she dabbled in black magic to get them on her side about her boyfriend, only it backfired and turned them evil. That wouldn't explain why she said the grandparents were free to tell the kids what she did if they wanted to, but I'm sure M. Night could find a way to explain it.