Sam had believed he could have done more, or done things differently in relation to his sons. When he communicated with Nina he began to appreciate that their sons had an agenda.
For some reason, which gained them neither monetary nor social benefit, they wanted to denigrate their parents. If Nina and Sam had not begun to communicate with each other, they would not know the complexity of the plot.
They would not know that their sons were gaining some sick pleasure out of ‘hurting’ them.
After some discussion, Sam and Nina decided to become the cold and distant and unconcerned parents their sons had made them.
The ‘boys’ were adults now, married, with their own families.
They didn’t need parents.