The Conqueror question

I am trying to find a book that I read probably 10 years ago. I remember it is a historical romance, I think set in the highlands. The hero is a knight gifted land by the king and a bride. There are two sisters and they trade places but then the one becomes jealous and poisons the heroine until till she loses the baby. While she is losing the baby the one sister is making it look like the hero was cheating on her. Not the best description but it's all I can remember about it, getting back into highlander books and wanting to re-read it.
Thanks