The following version of the book was used to create this study guide: Hepworth, Sally. The Good Sister. St. Martin's Publishing Group, New York, NY, 2020. Kindle AZW file.

Rose and Fern Castle are fraternal twin sisters who live in Australia. Rose is shorter with darker hair, and Fern is tall and slender, like their mother Nina. As the novel opens, the two are adults. Fern, who cannot handle situations of high sensory input and who struggles with social clues in many daily conversations, is a librarian. She is single and thrives on her routines. She has dinner three times a week with Fern and visits her mother in a nursing home every Thursday. Rose is married, but her husband Owen has recently moved to London to work. Rose says Owen left because of her desire for a baby, but she has complications that make pregnancy unlikely. As soon as Fern learns that Rose wants a baby she cannot carry herself, Fern sets out to find a way to have a child for Rose and Owen. As always when she is faced with a new situation, she does research and discovers that conceiving a child naturally is the most convenient way to fix the problem. That same day, a handsome American named Rocco Ryan enters the library. His hat makes Fern think of the Where's Wally books (Where's Waldo in America), and she nicknames him Wally. Fern asks Wally out on a date, and they picnic together. Within days, they have become close. Fern has almost forgotten about her initial plan to have a baby for Rose.

When Fern becomes pregnant, she turns to Rose as she would for any situation in her life. Fern knows she cannot be trusted to raise a baby, but she has become close to Wally and feels torn about keeping the baby a secret from him. Rose reminds Fern that she cannot be trusted because of a childhood event and because Rose constantly forgets to do things she promised to do. Wally, however, tells Fern that Rose is too involved in Fern's life and that Fern does not have the ability to see Rose as she really is.

The reader knows about the girls' childhood through Rose's journal, which details systematic abuse that went on for years. Several of Rose's entries have to do with Nina's boyfriend, Daniel. Daniel and his son Billy took Nina, Rose, and Fern camping for a week. During that trip, Billy and Fern had an ongoing competition to see who could hold their breath the longest. By Rose's account, Fern held Billy underwater to help him win the contest. Fern's account is different. She says Rose convinced her to hold Billy under and timed the event so that he was under only 40 seconds – not long enough to drown. In the one journal entry that seems to be honest, Rose reveals that Billy deserved to die and that Nina became so despondent that she began taking Valium. One night, Rose used her insulin to overdose Nina, putting her into a coma that lasted for years. In the present day, Nina was beginning to talk, thanks to a new therapy, and she says Billy's death was murder. She is also urging Fern not to hand her baby over to Rose. Within days, Nina is dead.

By the time Fern gives birth, she has begun to realize Rose is not a good person and should not have the baby. Fern reveals the truth to Wally who immediately steps in to help care for Willow. Fern's questions about her mother's death prompt an investigation. The police arrest Rose for murder. An officer shows Rose's diary to Fern, and Fern contradicts several of the entries. Fern discovers that Rose sought out Wally, who is a wealthy app designer, asking for money on Fern's behalf. Fern learns that Rose has perpetuated a series of other lies, including that Owen is away working. Owen actually moved across town and has no intention of reuniting with Rose. As the novel ends, Fern and Wally are settling into parenthood. Rose is in jail and is beginning a new journal, aimed at casting doubt on her guilt and making it seem Fern had a motive to kill Nina. Rose predicts that she will soon be raising Fern's baby and Fern will be in trouble.