Two Poirot novels prominently feature twin sisters as a central point of the mystery: The Murder on the Links and Elephants Can Remember. In other words, Christie’s second published Poirot novel, and her-second-to-last-published Poirot novel. (2, 2, 2…) The twin sisters of The Murder on the Links, as Poirot readers know, feature largely in the life and the future of Hastings. He marries “Cinderella” a.k.a. Dulcie Duveen, whose sister Bella marries Jack Renauld. Because of the South American interests of the Renauld family, the latter couple inevitably relocates there, and Hastings and Dulcie decide to join them as well to start a ranch.
There’s a strange little passage in Peril at End House when, as Poirot is getting on Hastings’ nerves, the following dialog ensues.
‘Do you suppose I’d have made a success of my ranch out in the Argentine if I was the kind of credulous fool you make out?’
‘Do not enrage yourself, mon ami. You have made a great success of it– you and your wife.’
‘Bella,’ I said, ‘always goes by judgment.’
‘She is as wise as she is charming,’ said Poirot.
Um, have Hastings and Poirot forgotten to which twin sister Hastings is married?
I don’t know if this has been written about or explained by Agatha Christie or anyone else, but it seems most likely that it is a mere mistake. If one is determined to resolve the problem “within the canon,” it’s just possible, I suppose, that Hastings’ sister-in-law is known to be the business brain behind all the entrepreneurial affairs of the family in South America, and he’s changing the subject to impress upon Poirot that even she trusts his judgment implicitly. But that is not really the context of the conversation. It’s a very bizarre moment.
It’s additionally funny– and weird– because in the television series, of course, one of the sisters is cut out altogether and Hastings and Bella Duveen do end up together!