Only in certain states of the US... But the world can indeed live without NC-17 Helen/anyone.
Considering other canon possibilities...
There’s always the Dowager Duchess’s line “The fact is, one never really visualizes one’s own son...” as snapped at Paul for apparently giving her a mental image she would have preferred to live without.
A quick search of Google informs me that the CofE’s 1936 Commission on Kindred and Affinity as Impediments to Marriage recommended allowing Harriet/Uncle Paul (tho’ I suspect that Peter’s will probably didn’t take it into account as one of the possible “contingencies”!).
Practically canon
Considering other canon possibilities...
There’s always the Dowager Duchess’s line “The fact is, one never really visualizes one’s own son...” as snapped at Paul for apparently giving her a mental image she would have preferred to live without.
A quick search of Google informs me that the CofE’s 1936 Commission on Kindred and Affinity as Impediments to Marriage recommended allowing Harriet/Uncle Paul (tho’ I suspect that Peter’s will probably didn’t take it into account as one of the possible “contingencies”!).