The Burden is the first of the books written by Agatha Christie under her Mary Westmacott nom de plume that I've read, although it was the last one published. It was published in 1956, the same year she published Hickory Dickory Dock and Dead Man's Folly.
This is a very strange little book centered around Laura, the least-loved child in her family. Very early in the book, she has a younger, favored, brother who dies, and Laura hopes that her situation will change. Her younger sister, Shirley, is born, which puts her back in the position of being less.
She makes friends with a cranky local man who generally dislikes children, but sees something in Laura that is interesting. He remains a constant friend and fixture in her life. Initially, Laura's feelings about Shirley are decidedly negative - until she saves Shirley's life at significant risk to her own, when Shirley is around 2 years old. Laura becomes deeply protective of Shirley from that point on, and raises her once their parents die unexpectedly.
Agatha Christie sets this up as a contrast between dark - Laura - and light - Shirley. Laura gets short shrift with her own life, dedicated essentially to caring for Shirley. Shirley falls hard for a pretty awful man, whom she later marries. This becomes a significant source of tension, when he ends up significantly disabled by polio. This does not improve him.
I read this book very quickly, partly to just get it over with, I think. I really didn't like it - the decisions being made by all of the characters were confounding. The ending was just weird.
Even in the context of this book, which is described as a "psychological romance," Agatha can't get away from crime. In addition, I don't think that I agree that this is a romance, as that genre identifier is generally applied today. I don't think that Laura is capable of an HEA, given the level of trauma that she sustained during her life (at least not without a lot of therapy). She is restrained to the point of isolation.
I'm not sorry I read it, because Agatha. But I am hopeful that it is the weakest of her Westmacott books, because it's hard for me to imagine that they could be worse.