But this is almost twenty years into the New Millennium! Assuming they are of legal age, of sound mind, the arrangement is consensual, and both brother and sister were in agreement, why are we still clinging to an almost pathological revulsion to incest? After all, this is a time when girls can be boys and boys can be girls (or anything else that a person might consider themselves to be along a non-binary, sliding scale of 'gender roles'); a time when women are free to make up their own minds, control their own destinies, and ‒ most importantly ‒ cast aside all those old social mores that had to do with the repression of their own sexuality; a time when the concept of the so-called 'nuclear family' is about as relevant as the Pony Express, iron lungs, or eight-track tapes; a time when a child born to an unwed/single mother is no longer an object of shame or sympathy; a time of same-sex marriages, open marriages, or even no marriages — merely an acknowledgement that 'A' and 'B' (and maybe even 'C') are now living together and are to be considered a 'social partnership'; a time when we are trying to eliminate tobacco usage, only to replace it with 'recreational use' marijuana; a time when "IF IT FEELS GOOD, DO IT" seems to be our new national motto.
This, I contend, is the surest indicator of the soundness (or the decay) of a society ... seeing those things which we used to condemn that we are now, whether for convenience or other purposes, willing to accept.

-"BB"-
(edited 3x for spelling, typos, and punctuation)