I believe they added incest to the plot to tie back with the daughter's frustrated and angry social commentary on women's contribution to British history as viewed by 1960's mores. In this view, disposability and anonymity dominate husbands', boyfriends' and fathers' regard of the women they brutalize. They sleep with their daughters because they can't recognize them as individuals. And if the patriarchy can't see them as individuals, them how can their history recognize the substantive contribution of women?
