When Anton Chekhov was talking about plotting in his plays he famously said, "If there's a gun on the wall in the first act, it'd better be fired in the third."
I just finished the book The Vanishing Act of Esme Lennox by Maggie O'Farrell in which there is a major "gun on the wall" that is aimed perhaps, but never fired. Before I started the book I looked through some of the reviews on goodreads and I noticed that one person, who hadn't liked the book, criticized it for having plot lines that weren't finished. For that reason I was sensitive to that aspect of O'Farrell's book. I'm fairly sure what the reviewer was talking about was the relationship between Iris Lockhart and her stepbrother, Alex.
There is no blood relationship between Iris and Alex. They lived together when they were part of a merged family. When they were younger they had a sexual relationship. Now, as adults, their relationship is much more complicated. It is true, as the other reviewer indicated, we readers never know for certain where Iris and Alex are headed. But the novel belongs more to Esme Lennox than it does to Iris Lockhart. This act of incest-in-law works as a wonderful comparison of the attitudes concerning behavior in the age when Esme was young and the age when Iris was young.
Ultimately the best stories are about the personalities of the characters. It seems to me that a "gun on the wall" tells us a great deal about the person who hung it there. And O'Farrell tells us about Iris Lockhart when she lets us know of her history with her step-brother. That's not only enough, it's the best a reader can hope for.
Check out my review of The Vanishing Act of Esme Lennox on goodreads.