I Spy A Demon by Keta Diablo

When twins Cecily and Calder Sizemore’s parents are killed in a car accident, they’re adopted by the Frost family—Gus, Mae and their sons, Marcel and Elliott. Over the years, Cecily’s love for Marcel evolves into anything but sisterly.
Cecily always knew something was amiss in the Frost household. Little things belied the calm, peaceful ambiance Mae did her best to portray. Calder tried to warn her things were not as they appeared, but she didn’t want to believe him. When Calder begs her to leave Des Moines, start a new life away from the secrets, away from the Frosts and away from Marcel, she takes his advice and her shattered heart and moves to Minnesota.
Now she’s been called home for her beloved brother’s funeral. There’s more to the story than meets the eye. Discrepancies in how her twin died lead her back to Des Moines, and back to Marcel―the boy who stole her heart, the man whose very presence turns her blood to liquid fire. Marcel has always kept dangerous secrets, but this time, Cecily is determined to uncover the truth about the Frosts… and the truth about how Calder really died.
She’ll find out what really happened to her brother, even if it’s her last act in life.