It certainly sounds like a woman did pull up in a car and tell the brother to get help. Whether that's all she did is debatable. Is it possible she told him to go to the further house in an effort to make him walk farther away from the fire for his own safety?
As for out of wedlock and waiting 4 years, to me either that might be when Jeannie began to strongly resemble her birth father in one scenario, or she was simply appealing to a kidnapper and it had nothing to do with her parentage. But an unplanned kidnapping and a fire on the same day do seem to be too much of a coincidence, especially to be pulled off successfully. On the other hand, starting a fire (and potentially burning an entire house down) as a distraction seems like overkill, if not unnecessary. First, there would be other ways to possibly get her (kids did play outside then), and secondly a fire runs the risk of attracting even more people and attention to the area. Possible, but gutsy to kidnap a child while fully visible/describable...one would likely only do that if she knew she would not be recognized by anyone, and would never be seen in the area again. Or the car for that matter, even if it was a rental.
Meanwhile, I wonder how big "fragments" are, because there would be more than say, coin-sized pieces left. Even on a child the femur bones would be longer pieces unless they were shattered pre-fire, which seems unlikely. Overall, it's more likely charred remains would be found, not bone fragments.