Kids are deff possessed.

SpOiLeRs.
Initially, I disregarded the hints of demonic presence, and thought that the twins were sociopaths.
Upon a second viewing, I feel it's hard to deny that the children are possessed. In the beginning, when the mother is looking for the ball, she says "i just saw a jackal". The jackal is symbolically tied to both the dead, and the devil. Their dog randomly whimpers outside the children's room while the camera is approaching. The parents check in on the twins sleeping, and within a second, they are suddenly up and standing at attention. The girl made the curtain rise without touching it. The twins disrupt their father's prayer, and the twins attack a boy with the name of Christian.
There are more instances as well. I actually wanted it to be that the children were sociopaths, because I'd find that idea more creepy/cool. I also agree with other theories that this demon has entered the camera. The children aren't possessed by the house... They moved to this new house trying to change the way the kids already were.