Factual error: The picture of the brakeline leaking is actually a fuel filter. The car would run out of gas. It would not have brake problems.
Factual error: In the scene where Ruby and Rhett walk into their bedroom and they find a Nintendo 64 on the ground, Rhett immediately sits down and inserts a game. Within seconds, he's playing a game without even logging in or choosing a level and so forth.
Answer: Terry told them when they first moved in that it was only going to be temporary so that they would think that they would eventually get separate rooms, but I think they always intended to keep Ruby and Rhett together in the same room so that they could easily monitor their movements. The only reason they quickly knocked up a second room (for Ruby) for when the social worker came, was to carry on the charade of being responsible guardians for them, and they knew if the social worker saw that a teenage girl and her younger brother were sharing, it would raise red flags, and they needed everyone to believe that they were responsible enough guardians, so that they could keep the kids and eventually get their hands on the trust.