The Devil of Helburn By the Sea by Rebecca Ruger

Sparks flicker when Julia Wake finds herself intrigued by the mighty Caelen MacFayden. Despite awakening unexpected feelings within him, their connection is hindered by cruel manipulations, leading to a shocking revelation. Caelen, believing he had shared a passionate night with Julia, discovers himself trapped in a forced marriage with her cousin, burdened by guilt over the unwitting role Julia played in this twist of fate.
Only distantly related to the stronghold’s chieftain and often overlooked, lacking the beauty and abundance of her cousin, Julia has guarded her heart fiercely, until the day she meets Caelen MacFayden. In the course of three days, he manages to tear down every wall around her heart, making her a willing accomplice to her own seduction…right before he weds her cousin.