A review by sharanyaaguha
The Cousins by Karen M. McManus

adventurous dark mysterious sad medium-paced

• Plot- or character-driven? Plot
• Strong character development? Yes
• Loveable characters? It's complicated
• Diverse cast of characters? No
• Flaws of characters a main focus? Yes

4.5

Everybody has secrets,” she says, taking a sip of her drink. “That’s nondebatable. The only question is whether you’re keeping your own, or someone else’s.

Then : Mildred and Abraham Story are the owners of a huge island and are super super rich. They have four kids Adam the oldest, Anders, Allison and Archer, the youngest. Abraham died when the kids were teen. One day when Mildred's kids were in college they received a note from their mother, which said they were no longer welcome in the island, and that they were disherited and must not keep any contact with their mother.

Now : Adam has a daughter, Aubrey who's a professional swimmer. Anders has a son Jonah, Allison has a daughter  Milly Mildred who was named after her grandmother and Archer is an addict.  One day the grandchildren receive a note from their grandmother welcoming them to the island to spend the summer. They all go even though they are not willing to. When they reach the island, Milly is suspicious that Mildred had no idea that they were coming. Milly grows more suspicious as Dr. Baxter, the story family's doctor starts acting suspicious. Soon they find out that their uncle Archer is the one who called them to the island and has been hiding there in plain sight. Dr. Baxter wants to tell Archer something (he recognizes him) and is soon killed. And Jonah who came to the island isn't Jonah Story but a classmate of his with the same name who was paid to come here (and also this Jonah's father was literally robbed by Ander Story when he was their accountant, so he came for revenge). Milly and Aubrey and later Archer knows about Jonah, but not the reason why he is here. 
Milly and Jonah start developing feelings for each other. On the night of the gala, Aubrey is called to meet their grandmother. Milly and Jonah go to the balcony and the moment they start getting intimate the curtain of the balcony is pulled revealing them to the entire crowd. That very moment Anders come from nowhere accusing Jonah, the rest of the cousins and Archers of committing fraud and trying to be good to his mother. But he is thrown out and so is Jonah. Jonah later returns to the island secretly and stays with Archer. 
In the meanwhile, the cousins learn that there was Matt who was Theresa (their grandmother's assistant) son and he was dating Kayla. But Kayla was in an on-off relationship with Anders, so he was very jealous of Matt. Allison had feelings for Matt and they get intimate once which later results in Allison's pregnancy. When trying to confront Matt who has been ignoring her ever since, she miscarriages. Anders and Adam then decides to kill Matt in a party by drowning him in the sea when he's drunk. Allison sees all these but never says anything.
Later, when Aubrey goes to see her grandmother again she spills tea on Mildred's hand. Mildred takes off her gloves and Aubrey notices she doesn't have her birthmark (which they both share in common). The next day she decides to go alone to find some evidence. In the meanwhile Archer gets a letter from Dr. Baxter which contains the autopsy report of Kayla. Now, Kayla was killed in a car crash because she was drunk but the autopsy report says otherwise, that Kayla was drugged. There's also a health report of Mildred showing that she suffered from the same heart condition that Abraham did and Archer is confused how Mildred is alive for 24 hours with the condition when Abraham died so soon. Archer then realizes that Aubrey's in danger and comes to save her. 
Aubrey is caught red handed by Mildred and held at gunpoint. When Archer comes, they are both taken to the sunroom and this is where Mildred or the one we thought was Mildred spilled everything. She knows that Matt wasn't in an accident but was killed by Anders and Adam, and Allison knew. And that Mildred died 24 years ago and she had planned the whole thing with Dr. Baxter and their lawyer. And also that she is Theresa. For 24 years Mildred has been dead and buried in her garden while Theresa pretended to be her, enjoying the luxury and cutting of everyone who would recognize her. She also killed Kayla because she was the one who told Theresa about Matt's death (after Anders confessed to her one night when he was drunk). She didn't know that Mildred was dead and Theresa took her place and was desperate to meet her. To silence her, they killed her.
Theresa wants to take revenge by killing Anders daughter Aubrey. But she fights her off with Archer and is able to hurt her. In the meanwhile Theresa has already ordered her assistant Paula (who we thought was Theresa but was actually her sister Paula) to set a fire (in a coded language which meant set the whole house on fire). So caught in the fire Archer and Aubrey escapes by climbing down the window with Theresa but she has already died and Paula is nowhere to be seen.  Their lawyer who was involved in the plan was jailed eventually, and Theresa and Dr. Baxter was anyway dead. Paula fled. 
The entire residence burns down and the children later realize that Theresa had never insured the place and has used every last bit of their money. So the money that the Story children dreamed off was burnt to ground as well. The grandkids then come back home and keeps in touch with each other. Jonah and Milly are dating. Archer went to rehab. Adam and Anders keeps denying their involvement. Allison re establishes her relationship with her brother, Archer.
And so eventually, everything falls to place, except for justice for Matt and Kayla because there was no evidence.