Three sisters (the eponymous aunts) take care of injured and sick creatures on a tiny island somewhere. They're not getting any younger, and when, one day, they’re watching a TV show about extinct animals, they suddenly realize that they might go extinct themselves. They need young people to help them and inherit their duties. Therefore, they decide to kidnap kids whom nobody will miss, as the island is top-secret, and just hiring people won’t do. (Their niece and nephew are spoiled children, who really wouldn’t be able to do hard work; the offspring of a sister who found the island unhygienic and fled as soon as she was an adult.) Etta kidnaps Minette, whose divorced parents are constantly fighting and have her travel from one to the other via train on a regular basis. Coral chooses Fabio, who is originally from Brazil, and is just about to be sent to a horrible English school, where it is implied he will be bullied for looking foreign. Myrtle soon decides that the boy she volunteered to babysit, Lambert, is a horrible child, but when he accidentally sniffs her chloroform, she sees no other solution than to take him to the island and let her sisters sort it out.On the island, Minette and Fabio, after proving themselves by hard work caring for the local fauna, are told about the island’s secret: There are magical creatures too, a family of mermaids, and a boobrie (a giant bird), among others. The aunts try to keep Lambert ignorant, but he manages to phone his father, and that’s when things become really difficult for the aunts.
Island of the Aunts contains examples of:
- "Blackmail" Is Such an Ugly Word: When Mr. Sprott finds out about the island and its magic creatures, he tries to blackmail the aunts into selling him the island; otherwise he will rat them out to the police.
- Boarding School of Horrors: Fabio is sent by his grandparents to the horrible Graymarsh Towers to learn how to be an "English gentleman", while he's mercilessly mistreated and bullied.
- Break the Haughty: Poor Queenie, after she's caught by Lambert's father.
- Broken Angel: After getting into an oil spill, the mermaids lose their mermaid-typical beauty.
- Broken Bird: Queenie's twin Oona, who was taken captive by a lordling on his yacht for several days before escaping. She is incredibly withdrawn and her voice is still hoarse from crying so much.
- The Bus Came Back: It's mentioned that the three sisters were originally five: there is Betty, who didn't like living on the island and becomes a suburban housewife in Newcastle-upon-Tyne, and Dorothy, an activist who travels the world trying to save rare animals from being served in restaurants. Both sisters eventually appear.
- Cassandra Truth: When Queenie is abducted by Mr. Sprott's henchmen her mother believes she has left with a lover. Her other daughter, who knows better since she was once kidnapped herself, keeps telling that Queenie didn't leave willingly but she's ignored.
- Cold-Blooded Torture: The villains use a family-friendly version of this to get information about the location of magical creatures.
- Corrupt Corporate Executive: Mr. Sprott is a billionaire who owns several businesses between the UK and America and has other non-specified shady activities that the police are investigating. He attempts to murder the aunts and the children along with them when they refuse to sell him the island.
- Dire Beast: The boobrie, a bird of the size of an elephant.
- Facial Composite Failure: The police circulate composite drawings of the aunts, who are wanted for kidnapping. However, the descriptions result in grotesque caricatures that look nothing like the real women.
- Family of Choice: Minette and Fabio find the aunts to be more caring than their actual guardians and become their actual Honorary Aunts. The aunts eventually write a will that legally leaves to island to Minette and Fabio once they pass away, despite the fact that they have two nephews of the same age, who are perfectly unfit to take care of the island and its creatures.
- Foreshadowing: At the beginning, the aunts are lamenting how much work there is to do on the island, and how some creatures refuse to leave even after they've been healed. It's because the Kraken has come out from his retirement and is heading to the island.
- Gaslighting: A somewhat more benevolent example than most. To keep Lambert from throwing a fit every time he sees one of the mystical animals on the island, Fabio lies to him that none of them actually exist and they're all hallucinations he's having as a side effect of the flour used in their food. Lambert accepts this and copes considerably better while thinking he's just imagining everything.
- Gender Flip: The traditional fairy tale of the selkie with the human lover is gender-flipped with Herbert, the selkie who isn't completely happy either as a seal or as a man.
- Gilded Cage: Once Queenie is kidnapped she's kept in the fancy Jacuzzi of Mrs. Sprott's bathroom. She's surrounded by luxury and beautiful things, but she's utterly miserable since she is captive to vile, greedy individuals.
- Half-Breed Discrimination: The main reason Fabio doesn't want to go to Graymarsh Towers; he's half-English and half-Brazilian, originally being raised in South America, and is bullied relentlessly by his English peers and looked down upon by his own paternal grandparents. The word itself is never used in the text, but it's quite obvious that all his tormentors are acting out of racism.
- It's All About Me: All the children's parents and guardians are self-centred and neglectful. It took all of them at least a week to notice their children had disappeared. Also, during the investigations they release interviews to earn some money out of their tragedy. Mr. Sprott is the worst of the bunch though, being a Corrupt Corporate Executive who blames his attempted murder of Minette, Fabio, and the aunts on them for not "cooperating" with him and his evil deeds.
- The Kindnapper: The aunts are well-meaning, and originally intended to only kidnap kids who wanted to get away from their ordinary lives. Lambert was an accident.
- Kraken and Leviathan: The giant Kraken puts in an appearance; you'd be forgiven for not envisioning a traditional Kraken as you read, though, since cephalopod-like traits aren't mentioned even once in the whole book. There's a little one, too; it likes cookies and is quite adorable.
- Lima Syndrome: The aunts were never malicious, but they do grow fond of the kids and become a bit more realistic about the whole kidnapping business.
- Loophole Abuse: How the aunts are acquitted at their trial. The defence barrister argues that the by-the-book definition of kidnapping is "holding someone against their will". Which is not exactly what happened, because the children enjoyed staying there and the aunts would have let them go if they wanted.
- Never Mess with Granny: Never mess with mermaid granny. The aunts are not actual grandmothers, but still badass, too.
- Our Mermaids Are Different: Of the mermaid family, one is a visibly old lady, one is an chain-smoking divorcee, and the baby is a merboy, who is rather chubby. Only the young twin mermaids resemble the stereotype.
- Parental Neglect: Minette's parents, towards her. While her mother's boyfriend isn't technically related, we do see him hoping she'll be sent to stay with her father because she takes up too much space in their house.
- Also it takes eight freaking days to realize their daughter has disappeared. While it is easier to lose track of a child living between two houses, any sane divorced parent would check the same evening if their daughter is arrived to destination or ask the other parent for explanation instead of buying the words of a stranger.
- Police Are Useless: Inverted. Minette and Fabio's families believe that the police are sitting down doing nothing because they want to blame someone instead of themselves (the children have disappeared mostly due to their neglect) and because they are profiting from releasing interviews where they lament about the investigations and get paid for them. Mr. Sprott also resolves to go find Lambert by himself when he receives his call instead of alerting the police because he doesn't trust them (he's a Corrupt Corporate Executive and the police are investigating his shadier businesses). The police are actually doing a fairly good investigation following Mr. Sprott himself, but most of the time they are staying silent until they find real evidence.
- Selkies and Wereseals: Myrtle, the youngest aunt, is friends with Herbert, a suspiciously human-like seal. She reads him poetry and plays him classical music with her cello.
- Shapeshifting Lover: When Sprott's bodyguard Des attacks the abovementioned seal, this trope ensues for the aunt who was friends with his seal-form.
- Ship Sinking: Aside from the actual ship sinking Herbert and Myrtle do not get together despite an entire novel of Ship Tease. He's not happy as a man and decides to turn into a seal again and leaves with the Kraken. Myrtle is heartbroken, but knows that letting him go is the right thing.
- Sirens Are Mermaids: Parodied. A mermaid tried to make money by causing a ship to sink, enchanting the captain with her hypnotizing singing voice... sadly, the times have changed, and instead of gold and treasures, the ship she caused to sink had oil on board.
- Stockholm Syndrome: It doesn't take the children long to enjoy living on the island and helping the aunts. It helps that the aunts are considerably more caring than the children's own guardians are. This is lampshaded at one point when Minette asks Fabio if they shouldn't be planning an escape from the island, but they enjoy living there too much to think about running away.
- Would Hurt a Child: Mr. Sprott invades the island to kidnap the creatures and create a horrible amusement park that will exploit their suffering for cheap laugh. At the end of the book, he's about to kill everyone from the island—including two children—by throwing them into the sea to drown.
