In chapter 1 Sir Thomas has his qualms about letting Fanny come to stay " he thought of his own four children, of his two sons, of cousins in love" (p.4 in the Penguin popular classics ed.). Wasn't marrying cousins extremely wide-spread in Regency England? Is it only her low social status that Sir Thomas is afraid of or was there some awareness that marriage withing the family could cause i.e. medical problems? When did the costum stop, when was is forbidden by law and is it still legal in other countries today?
( I've posted this before but probably in the wrong place)