Agatha Christie’s works have been typeset many times. It could be a printer’s error.
However, I’m inclined to think that the passage is being related by a third party, possibly in a letter, or in a description of a diary entry, or in some other form where the style can be both casual and indirect. It is possible that this “frame of reference” has been established some distance “upstream” of the sentence in question.
For example, it is possible in English to refer to oneself in the third person, in order to create a slightly distanced and ironic tone.
“Jimmy was playing in the sandbox, Ellie was on the swings, and their father here was spraying water on Uncle Bob and his son” (points to self).
Thus, Ellie - and her father here - forgetting the sandbox - Jimmy - he said that we really “had to see his castle - and Stop Hogging All the Water”. So we did.
In effect, the narrator is “getting lost” in the complexity if their narrative, but getting back on track with a clear sentence at the end.
I don’t know if that was Ms. Christie’s intent here, but it would not be unlike her to write this way.