The Eldritch Conspiracy

Celia Graves was once an ordinary human, but those days are long gone. Now she strives to maintain her sanity and her soul while juggling both vampire abilities and the powers of a Siren.
Not every bride needs a bridesmaid who can double as a bodyguard. But Celia's cousin Adriana is no ordinary bride: she's a Siren princess, and she's marrying the king of a small but politically important European country. She's getting death threats from fanatics who want to see the whole Siren race wiped out — including Celia herself, who is half Siren.
Luckily, Celia is on duty when a trip to a bridal salon is interrupted by an assassination attempt, so everyone survives. When Adriana returns to the Siren homeland to try to prevent a coup, Celia is free to hunt for the terrorists and the vile mage who is helping them (while keeping her eyes open for the perfect maid-of-honor dress).

Assuming the bride and groom both live to see their wedding day, this will be one royal wedding no one will ever forget.