The part that tipped me off the mother (Spoilers)
Wasn't the antagonist was when the boy came in her room while she was asleep then he left and the mother crunched down on something, we then see she was eating a sandwich or toast and had it hid under her blanket. Now in the trailer i remembered a cockroach crawling in her mouth and then seeing her crunch down on it and eat it.
But as soon as the sandwich was revealed thats what she was crunching on i realized what the trailer had done, by clipping different scenes together making you think something was wrong with her. So from that point on i knew it was gonna be the boys was paranoid and it was gonna be that she was their mother but they was so paranoid they was gonna do something stupid.
I hadn't figured out about the one twin actually being dead and Elias was only seeing him in his head, but when she ate that sandwich it kinda signaled to the viewers she was gonna turn out to be perfectly normal and the boys was gonna be the antagonists.
Thats what i love about these high school girls, i get older they stay the same age.