Great parents. No one knows why their one child out of 5 went wrong. They did the same thing 5 times, and failed once. Not their fault.
I think it's a GREAT MISTAKE to treat all siblings
EQUALLY.
CHILDREN ARE DIFFERENT. Certainly one has to be fair. But also, one has to teach that life is not particularly fair--deal with it.
In any case, some children require 2-5 times what maybe all the rest of the children required put together.
I can say that theoretically I could agree that such COULD happen.
I just KNOW that personally, in observing more than 5,000 families/children at significantly close range, I HAVE NEVER seen it such that the parents REALLY DID even mostly ALL THEY COULD HAVE DONE to bond, connect, train the child.