The Sin Eater's Daughter by Melinda Salisbury

Even though she lives in a castle and is engaged to the prince, nobody looks at or speaks to young Twylla. That is because poison runs through her veins, and just a single touch from her will kill, unless you have royal blood which grants you immunity. Twylla never questions her role to the queen or the gods until a new guard is assigned to her, a young man who doesn't see the cursed executioner, but the girl hiding behind her title.