] So as Emma and Isabella's children would be double first cousins with twice the usual amount of genetic material in common. They would hold 1/4 of their genes in conmmon and be four times as likely as unrelated people to have unhealthy offspring if they had children. Not to mention the fact that for this reason their marriage would be specifically forbidden in North Carolina! (Although apparently not forbidden in the UK)

Any children of Edward and Elinor Ferrars are almost that closely related to John and Fanny Dashwood's son (and any other children)--except that John is only a half brother to Elinor.