SOLVED. YA Fantasy book about an island protected by merpeople and there's an annual breeding season with the humans for them to continue their species. [s]

[Tiffany]

I'm looking for a book where after a girl's mother dies, the girl goes to an island which she found something related to in her mom's stuff. The island is actually protected by some kind of mer people. If I remember correctly, they were some kind of spawn of poseidon. She also finds out there's some like annual breeding season where there's like a sort of ball. The girls dress up and the mer people choose a girl to breed with. If I remember correctly, the mer people can't breed among themselves so they needed to breed with the humans.

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Later, it is revealed that actually the girl's mom was actually from the island and breeded with a merperson. But, she didn't want to give up her child. So, she escaped. But, the girl actually turns out to be a special mer person aka a siren which makes her extremely powerful. She didn't realise this before cuz the mer people attributes don't affect her when she's far from the island. So, she looks like a regular person. But, when she's on the island, she starts to experience the attributes but doesn't realise it. And then after the breeding thing or sometime around then, she realises what she is.
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I read this book some time around 2012 to 2014, but it's probably much older than that.

[Angela]

Revel by Maurissa Guibord?

From School Library Journal:

Gr 9 Up — After Delia McGovern's mother dies of cancer, the 17-year-old has to seek out a grandmother she has never met, who lives on a mysterious island off the coast of Maine. Trespass Island doesn't appear on maps, and the residents forcibly discourage outsiders. When Delia manages to make her way to the island , she discovers that it is home not just to human beings, but also to sea-living descendants of the Greek god Poseidon called Aitros, or First Ones, who rule over the humans. Prevented from leaving, Delia makes friends with other young people and with Jax, a young male First One who is an outcast from the Aitros clan. However, she finds that the protection of the First Ones comes at a price. At Revel, the summer solstice ceremony, young human women are compelled to mate with male First Ones and give up any resulting infants to them. When Delia resists, she makes a terrifying and amazing discovery about her true nature and her mother's reasons for fleeing the island .

[Tiffany]

Wow, that was fast! That's definitely the book. Thanks a bunch!