I have the same trouble with the apparent phone conversation with the sister and the time the fire was reported, as well.
Initially I thought there was a chance that the fire began accidentally, maybe Sharon was able to wake Terry up or help him get out of the house, and he or she or both were injured. They still were able to get into their truck and drive off for help- then maybe accidentally went off the road. If that happened, I honestly think they would have been found fairly quickly because I'm assuming there's probably only one road they would have taken and that was probably searched.
Just do not know a darn thing