Sandra and Neville married young and had two sons, Paul and John. They were poor and tried, but failed. They thought they were doing the right and responsible thing in sending their children to his mother.
They sent them to save them from virtual starvation, and protect them from the souring of the marriage. Sandra and Neville assumed they would work it out, get on their feet, and then, in the near future they would regain their children.
This did not happen.
Olga wanted the two boys and somehow they were turned against their parents. Somehow, P & J learned how to catch her feelings and turn her against her two children, (the brother and sister of their father, Neville).
Why and how they did it can be debated, but for some reason they seemed to realise that Olga loved them and would take their side, no matter what and they bathed in this evil power.