Christie's last three novels are all cold cases, and they're all pretty bad. I wondered why Christie went back to this tired old formula so often. I believe it's because she knew she was no longer able to create a traditional crime that could (in principle) be solved by the reader through clue detection. Therefore, the rambling recollections where the detective is essentially handed the solution (after painfully boring pointless meandering) that defines these last three novels. 
 She's also obsessed with memory. Ian Lancanshire suggests that AC was aware of the onset of dementia and used the book as an outlet for her frustrations. This theory is supported by the fact that comparing a concordance of this book against concordances of her early books shows a sixfold increase of indefinite nouns like "thing". But the problem is that her handling of memory is absurd. The notion that an adolescent wouldn't remember exactly where she was when she learned of her parents' double suicide is absurd. Throughout this book, characters forget things you'd expect them to remember. And other characters remember the most ridiculous details that not even an elephant could recall under the most careful interrogation. 
 But what makes the book unreadable are the outright contradictions (based on the surviving daughter's age we must conclude that the deaths were 12-14 years ago, but the years 1952* and 1973 specifically mentioned in the story would make the deaths 21 years ago), gross misremembering (were General and Mrs Ravenscroft 25 years apart or did they meet when the general was a young man?) and boring repetitions as we're guided through this maze of recollections. Often you are led to wonder whether Christie, in addition to the characters, has forgotten that a particular topic was already covered. And to top it all off, there's only one suspect. So there's not even an opportunity for the reader to be misdirected. I guess that's marginally better than the zero suspects in Postern of Fate. The solution depends on the sisters dying three weeks apart, a striking fact that isn't discovered until the last 30 pages. You'd think that somebody who remembered that the victim ordered two wigs 12 (14? 20? who knows?) years ago would mention something like that. 
 Bottom line: This book is undeniably worse than Nemesis or Sleeping Murder (both of which I gave 2 stars) and can only be fairly described as marginally better than Postern of Fate (1 star), published the year after this book. If ever there was a need for 1.5 stars, this is the book for it. I ultimately went with 1 star, although that may be unduly harsh. 
 * I understand (e.g., wikipedia) that 1952 was changed to 1960 in later editions to resolve this contradiction.