As far as incest goes, I don't think that Barnabas and Carolyn are closely related. He was born in the 1700s; She is the descendant of a cousin of his, and the way they spoke of Millicent and Daniel in the 1795 story, it didn't seem that they were that closely related either. They were probably 2nd or 3rd cousins twice removed. Now, if you look at their relationship as being what Barnabas said it was, he is a cousin from England; a descendant of the original Barnabas Collins, they are very distantly related. Someone should do a family tree of this supposed cousin from England just to show how distant the relationship is; remember, Joshua wanted his brother Jeremiah to marry cousin Millicent. I seem to recall that the characters implied, or maybe they even said that Millicent wasn't a first cousin. I know that it wasn't frowned upon to marry first cousins even in the Victorian age, and this was even further back than that. I'm not sure when people figured out that you probably shouldn't marryyour first cousin, or when the law started to prohibit the practice.
Misa