What do the children learn about the roots of their fathers behavior?

The children suspect that their grandmother abused their father in the same way she abused Brian.

It was gross and creepy to think about, but it would explain a lot. Why Dad left home as soon as he could. Why he drank so much and why he got so angry. Why he never wanted to visit Welch when we were younger. Why he at first refused to come to West Virginia with us and only at the last possible moment overcame his reluctance and jumped into the car. Why he was shaking his head so hard, almost like he wanted to put his hands over his ears, when I tried to explain what Erma had been doing to Brian.