"It was so complicated. So difficult. And I... I said she was in hospital. And I know it wasn't true. But once I'd said that... I couldn't... I couldn't change it. Do you understand... Christopher...? Christopher...?" (157.42)
This just reinforces the sinister nature of lying – that once you start lying, it's really difficult to back out. Christopher's father expects him, of all people, to understand. We'll also point out again how conflicted he is about just how he ended up telling his son this awful lie.