Visible crew/equipment: When Josh goes to 'The Further' to look for Dalton and goes up to the attic where the red door is, the room is filled with an eerie smoke, but as the camera pans out a little, you can see the smoke machine in the bottom left hand corner billowing it out. (01:18:50)
Visible crew/equipment: When the dad goes to wake up Dalton, he bends down to get some clothes for school, and you can see the audio wire.
Answer: It's implied that the girl went crazy and killed her family, but it's not really elaborated on. They're just some of the many spirits to be trapped in and haunt the Further, likely due to the violent nature of their deaths.