Not what I expected at the end...
Two thirds of the way through this movie I was convinced that this movie was about a folie à deux (shared delusion) brought about by the brothers having been sexually abused when young (with the younger brother being abused after the older one was sent away).
Firstly, I thought the premise was too silly to be taken literally in a non-comedic horror.
Secondly, the monster was particular associated with bed, where you might expect abuse to have taken place.
Thirdly, when the monster's hand touched the young brother he said it felt cold and that it made him want to go to sleep - perhaps like being chloroformed?
Fourthly, their nightmare scenes sometimes involved a demonised incarnation of their father, suggesting the father was the abuser; although at one point I thought the overly friendly neighbour might be the cuplrit.
I figured that in the end the truth would come out and the monster would be 'put to bed', so to speak (pun intended).
So I was quite surprised in the final act when it revealed that, no, the monster was literally real. What happened to enable the monster to come out and slaughter everyone? Why hadn't it done so before now? When the monster was manifesting in the neighbour's house, who was wandering about under a sheet? Why would the mother's ashes defeat the monster?
Dumb movie!