What if... (SPOILERS)
she really was crazy ???
Maybe she did imagine the kidnapping story...she put her sister under the house at the point where she saw she was "parked in" and went back into her house and her sister said she could have the keys to her car...after all, instead of shooting the bad guy, or leaving him down the hole while she got the police, she destroyed the evidence (with fire) and then told the police nothing had happened, ensuring no investigation.
End result is her sister and those around her now believed her story about the original kidnapping...so now she isn't thought of as "crazy".
The only thing that would be hard to explain would be the photos she then sent to the police at the end...as that would mean that SHE had, in fact, done similar to the other girls...and would have to have photographed herself bound...but it's possible...
Just a thought ;-)
I'm a Lieutenant Detective in, uh, Homicide. That's a fancy name for murder.