I can't remember the name of this romance book. Help anyone?

It's a historical romance book and I think it's either by Johanna Lindsey or Amanda Quick. It's about this guy who has to go to this castle and comes across this peasant, who is actually a girl, who he thinks is a guy, to go to this castle and marry one of the daughters of a duke. He practically makes the girl who is a tomboy change into this wonderful woman. But the girl is one of a set of twins, but they are complete opposites, but still get along easily enough. If someone could help me with this it would be greatly appreciated.