In 1831 Shelley makes one of the most controversial revisions of the 1818 edition. In the 1818, Elizabeth is a blood relation--Victor's first cousin--since she is the daughter of his father's brother. In the 1831 edition, Elizabeth is instead "discovered" by Caroline. Caroline notices a comely blonde girl "of a different stock" among the dark-eyed, Italian "vagrants." When Caroline learns that Elizabeth is the daughter of a Milanese nobleman and a German woman she decides that Elizabeth and Victor should someday marry. Thus the potential implication of incest in 1818, when Victor eventually weds his cousin, is erased in 1831.

See, for instance:

Ketterer, David "Thematic Anatomy: Intrinsic Structures" in Frankenstein: Bloom's Major Literary Characters, ed. Harold Bloom (Philadelphia: Chelsea House, 2004), 33-54; Richardson, Alan "Rethinking Romantic Incest: Human Universals, Literary Representation, and the Biology of Mind." New Literary History 31, no.3 (2000): 553-572; Twitchell, James B. Forbidden Partners: The Incest Taboo in Modern Culture (New York: Columbia UP, 1987).