When the kids go upstairs with the flames after summoning them and disappear from the screen, there is a clear cut to separate shot before the aunt and uncle are shown going after them, despite it looking like it's a one whole shot.
When the car with the corpses is pushed down the cliff, it's obviously empty.
The children know that a stair removed itself and that's why Mr. Coby hurt himself, but they couldn't have known that nor seen that.
Mother's ghost tells her daughter that they wouldn't hurt anybody, yet they hurt Mr. Coby and the spiritualist.