The Highland Heir by Allison B. Hanson

Kieran has spent his life living in the shadow of his late older brother. Brody had been the heir his father had loved and wanted, while Kieran was born to a woman his father despised for her betrayal. Kieran has only ever felt the bond of family with his best friend, Ella.Ella knew one day Kieran would be called on to do his duty to his clan and marry for an alliance. But she never expected his engagement would suddenly force her to see him in such a different way. They’d known each other all their lives, but aside from one exploratory kiss, she had never been attracted to him as a woman is to a man. Until now.
As the time comes for Kieran to marry, he realizes he has feelings he shouldn’t have for the lass who’s been by his side all his life.When the marriage contract is breached, Kieran hopes it means freedom for them to be together, but instead, it leads to war and a secret that will change their lives forever.