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: My theory is that Terry deliberately acted like a creep to make Ruby uncomfortable around him. If she accused him of being inappropriate, he could deny it, making her look like the "troublemaker." She already has a history of being in trouble at school and sneaking to parties/raves. And he set her up for another accusation of plagiarism. He probably wanted to get rid of her because, unlike her brother, she was not fooled by him and Erin. He later points out that they only need one of the children in order to get the money.