Cain was the son of Adam & Eve according to the first book of the Bible, Genesis. Cain would later kill his brother Able out of jealousy when God favored Able's sacrifice over Cain's.
Stan and Steve read the Bible together in "Daesong Heavy Industries" when Steve challenges the idea of religion. As the read about Cain and his wife, Steve points out that they would have to bother be related as Adam and Eve were the only other previous humans. Stan tries to brush it off as guys like Cain always getting the girl, although he stops and admits it still doesn't make sense.