Revealing mistake: When Henry's mom is pushed off the cliff her cable is visible.
Visible crew/equipment: When Henry helps pull Mark onto the treehouse at the beginning of the film, if you look very closely to the left you can just make out a crew member dressed in black helping him on to the platform. The crew member then ducks back under the platform.
Revealing mistake: After Henry falls dead on the rocks, a black cable is visible that is attached to the supposed corpse of henry and to another rock ground level, possibly to stop the dummy corpse from drifting away.
Answer: Henry caused his younger brother's death by drowning, just like he was planning to kill his sister, Connie.
raywest ★