I've just reread this story for the umpteenth time, and only now have realized a major problem with the plot (SPOILER!!!!). The story has two young stepbrothers - Ronald, the grandson and heir of Tom, and Timothy, his stepmother's son - both almost the same age, brought up together and good friends. According to the end, Beryl, Timothy's mother and Ronnie's stepmother, has tried to kill "Timothy" because he is really her stepson Ronnie, Tom's grandson and natural heir, and she wants her real son, "Ronnie", to inherit. However, she has already switched Timothy and Ronnie when they were babies, presenting Ronnie as Timothy and vice versa. That means that at the present point in the story, The young man who is known as Ronnie, and is going to inherit, is really her son Timothy. So why does she need to kill "Timothy" (who is really Ronnie)?
The only reason I can think of is because "Timothy" (like his real grandfather Tom) is colour blind, besides resembling his real mother, Tom's daughter. There is always the danger, as soon as people realize he is colour-blind, that they will connect the dots and suspect a substitution. However, AC doesn't even hint at that as a reason - she just presents us with the motive of having Beryl's real son inherit, which he is going to do anyway as things stand.
So can anyone tell me what I'm missing here?