Ian's assumptions were partially right, so I'll just add to that because I don't think I'll be explaining it in future chapters.
He wanted Ian's daughter for his son because she'll come with a hefty dowry, and some land, especially if Ian doesn't remarry and produce more children. Plus, her mother was a Brodie, which that in of itself is backed with more money, not to mention influence in Scottish society and monarchy. Another reason he wanted Fia for Colin, is just to spite Ian, take away and have something that Ian really loves. In a way, he probably knew that Ian wouldn't go for it, and it was just another reason for him to attack Ian and his people -- although in his head, it's a more legitimate reason, one that his people wouldn't question, he also won't have to worry so much if he gets caught.