The Good Sister (2021) is the seventh novel of bestselling Australian writer Sally Hepworth. A work in the domestic thriller genre, The Good Sister combines complex family dynamics and psychological suspense with a mysterious plot that has a murder at its center. The novel is the story of twins Fern and Rose Castle and the extremes to which one sister will go to get what she desires from the other. Told through the varying points-of-view of Fern and Rose, the novel uses two seemingly unreliable narrators to build a gripping, suspenseful story. The Good Sister is also notable for its sensitive portrayal of neurodiversity. This study guide uses the Hodder and Stoughton 2021 Kindle edition.

Sally Hepworth is a New York Times bestselling author whose novel The Mother-In-Law (2019) is being developed as a TV series. Hepworth’s books often explore the dark side of family dynamics while maintaining a deep sense of empathy toward characters. Other works of hers include Darling Girls (2022) and The Soulmate (2022). Hepworth lives in Melbourne with her husband and three children. Her books have been translated into 20 languages.

Content Warning: The text and this guide discuss child abuse, murder, sexual abuse of a minor, fertility issues, and emotional abuse.

Plot Summary

Fern and Rose Castle are 27-year-old fraternal twin sisters who live in Melbourne, Australia. Petite and dark-haired Rose is an interior decorator married to Owen, and tall and slender Fern is a librarian. Rose has type I diabetes and Fern has issues with sensory processing. Their mother Nina suffered brain damage from an overdose of Valium when they were 12 and has been in a care facility ever since. At the start of the novel, Owen has moved away from Rose to London, ostensibly because of her struggles with fertility. Fern and Rose are extremely close and dine together on Mondays, Tuesdays, and Thursdays. Fern visits Nina weekly, but Rose never visits Nina since Rose believes Nina was a negligent, cruel mother. Rose goes into therapy to deal with Owen leaving her and begins to note down her past trauma. When Fern discovers Rose may not be able to have a baby, she secretly decides to get pregnant so Rose can adopt the child. Rose goes to London to spend four weeks with Owen.

Fern meets a young American man, Rocco Ryan, at the library. She refers to him as Wally because he reminds her of Wally from the Where’s Wally books (Where’s Waldo in the US). Fern asks Wally out for a date with the ulterior motive of getting pregnant. Unexpectedly, she discovers a kindred spirit in him, and the two grow close. Wally too has sensory processing issues and anxiety. In the US, he sold a restaurant delivery app for millions of dollars, and he is in Australia to make a fresh start. In Rose’s absence, Fern begins to form new relationships, especially with her sympathetic coworkers at the library. Away from Fern, Rose worries about her since she believes Fern is too gullible. Rose returns early from her trip when Fern tells her about Wally. The sisters have a brief falling out over Rose’s overbearing attitude toward Fern. Meanwhile, Wally begins to suspect Rose is too involved in Fern’s life.

When Fern becomes pregnant, she asks Wally if he likes children. Wally says no, assuming her question is theoretical. A devastated Fern breaks up with Wally. Fern feels she cannot handle the pregnancy on her own and reaches out to Rose. Rose suggests Fern have the baby for her, and Fern agrees. Despite the commitment to Rose, Fern is unhappy about Rose’s intrusive questions and clinginess. Fern feels indebted to Rose because of their traumatic past. When they were 12, Fern accidentally drowned a boy named Billy. Rose covered for her, saying Billy had drowned by getting tangled in the weeds. Rose has taken care of Fern ever since Nina overdosed and was put away in a hospital. Nina was a negligent mother who took her anger out on Rose. However, most of Fern’s memories of Nina are good.

Fern visits Nina in the hospital and tells her about her pregnancy. Nina tells her not to give the baby to Rose. Fern is puzzled. Nina’s speech therapist notes she has been muttering something about Rose and a boy called Billy. Fern tells Rose she is worried about Nina. A few days later, Nina dies unexpectedly. Fern learns Rose visited Nina the day she died. She is initially happy that Nina and Rose reconciled; however, she grows suspicious of Rose when Wally shows up at the library and tells her Rose has been trying to extort money from him on the pretext of Fern being pregnant and broke. After Wally leaves, Fern begins to experience contractions and rushes to the hospital. She calls up Rose, who helps her through the labor. Fern gives birth to a baby girl. Rose tells her she is not getting back together with Owen and will be raising the baby herself. Having learned new information about Rose, Fern is not sure she wants her to raise the baby. Fern escapes with the baby from the hospital. Police take her in custody after Rose lodges a complaint.

It turns out, Fern cannot be charged with kidnapping her own baby. Police tell her they are investigating Nina’s death, as the cause of death is not known. They have found puncture marks on Nina’s scalp, suggesting she was injected with something. Fern begins to connect the dots about Rose. It was Rose who visited Nina last and injected her with insulin to kill her. The secrets about Rose come tumbling out. Owen did not move to London but merely the other side of town. He left Rose because of her abusive nature. Nina did not punish Rose. Further, Rose planned for Fern to drown Billy. She suggested to Fern to hold Billy under water so he could beat her diving time. She would tell Fern when it was 40 seconds so Billy could come up safe. Rose had exceeded the time deliberately, knowing Billy would die.

Wally and Fern reconcile. Wally is happy to have a child. They name their daughter Willow. Rose is arrested for murder. In jail, Rose begins a new journal, which is meant to implicate Fern for the murder of Nina. Rose notes that once Fern is arrested, she will be raising Fern’s baby as her own.