The Good Sister by Sally Hepworth, A mystery-thriller told from the perspective of two twin sisters who were raised by a sociopathic mother.
In The Good Sister, Rose and Fern are twin sisters. Fern works at a library and uses routine to keep her life in order. Meanwhile, Rose is an interior designer who has been trying unsuccessfully to get pregnant.
When Fern comes up with an idea to try to help Rose, it leads down a path that will disrupt both their lives and unearth old secrets.
(The Detailed Plot Summary is also available, below)
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The one-paragraph version: Rose and Fern, 28, are twin sisters who had a bad childhood with a cruel mother. They are very close because Fern is autistic and very reliant on Rose. When Rose is unable to conceive, Fern offers to step in as a surrogate, meets a man named Wally and gets pregnant. Just before the baby arrives, their brain-damaged mother dies. It's then revealed that Rose, due to her narcissism and jealousy, has been manipulating Fern all along into believing that she's incapable of taking care of herself and a danger to others. Rose lied about their mother being cruel, caused her brain damage and killed her. (When they were kids, Rose also murdered a boy who liked Fern and made Fern think it was her fault.) By the end, Fern is happily raising the baby with Wally. But as the book closes, Rose is trying to come up with a new set of lies to incriminate Fern.
Rose and Fern are twin sisters who had a difficult childhood with a cruel mother. Rose helps to take care of Fern, who is on the autism spectrum and has trouble remembering things. There were incidents where Fern has hurt people in the past, including one in particular where Fern held a boy named Billy underwater until he died.
Similarly, when Rose goes to visit her husband Owen (who is in London on business), she asks Fern to watch her dog. But Fern forgets and the dog nearly dies.
Meanwhile, Rose has been having trouble conceiving, and Fern offers to step in as a surrogate. At the library she works at, Fern meets a man named Wally for that purpose, but then Fern and Wally genuinely fall for each other. Wally turns out to be a multi-millionaire computer programmer. Soon, Fern is pregnant. Fern is temped to raise the baby herself, but Rose reminds her of how dangerous she can be for others and how she's not fit to be a mother. Rose convinces Fern to break up with Wally instead of telling him about the baby.
For the last 16 years, Fern has been visiting their brain-damaged mother regularly, though Rose never does. When Fern tells her mother about the surrogacy, her mother tells her not to give Rose her baby. Soon, her mother dies. Fern learns that Rose visited their mom the day before. Fern also learns that Rose has asked Wally for money.
Fern has the baby who she named Willow, but is having misgivings about Rose. She leaves the hospital early to get away from Rose, but the police soon show up, take her baby away and admit her to a psych ward.
Eventually, things are cleared up, and it's revealed that the police are investigating Rose for the murder of their mother. Rose is diabetic and she gave their mother an overdose of insulin injections to kill her. (Rose did the same thing when they were kids, which is how their mother ended up brain damaged in the first place. Rose had been jealous that their mother preferred Fern.)
At the hospital, Owen shows up and tells Fern that he actually left Rose and that she's been making stuff about him because she's a narcissist. Wally also shows up and is happy to learn he's a father.
It's also revealed that the Billy incident was the result of Rose's manipulations. Rose was jealous that Billy liked Fern instead of her. Fern and Billy had been playing a game competing over who could hold their breath the longest, but Fern kept winning. Rose suggested to Fern that she hold him down so he could win and be happy. Fern researched how long would be safe, but Rose was the one watching the time and purposely had Fern hold him down for too long, and he died.
As it turns out, all of Rose's parts in the book are lies, which were written in order to establish a foundation to try to prove that Fern is incapable of being a mother and to incriminate her in the death of their mother. Fern is shown Rose's journal, and she sees how Rose has twisted all these situations around to paint herself as a victim. In reality, their mother was not cruel; Fern's memories are positive (but Rose was always telling her she just misunderstood situations). Also, Fern is not forgetful (Rose just makes things up and says that Fern forgot to make her think that).
By the end, Fern is happily raising her baby, Willow, with Wally. Fern is also learning that there are people other than Rose who are willing to take care of her and accommodate her.
As the book closes, Rose is now in prison awaiting trial for the murder of their mother. As requested by her psychiatrist, she writes out the truth of what really happened. However, then she tears it up and starts a new journal, one with more lies to implicate Fern. She plans to tell them that Fern asked her about how to administer insulin, that Fern always resented visiting their mother, and how Fern asked to borrow her bracelet (e.g. she's going to say that Fern pretended to be her when visiting their mother before her death) ...