Let’s say some global catastrophic event was to happen that wiped out all of humanity leaving only one man or woman left theoretically that were brother and sister.
In order to propagate the survival of the species would incest under those conditions be alright?
I might suppose that if those two [were] the only ones left, incest would not even be an issue here. If their only real desire was to re-propagate the world, - a form of love and in this case, the most instinctive drive, and not sexual pleasure, well, one might say, they would be more in tune with being an Adam and Eve, not a brother and sister. Love is that which is beyond good and evil - so incest would cease to be an issue 'within their minds" being that they would have more urgent and paramount issues to contend with - namely, populating the world again. They might even rationalize, and with just cause perhaps, that this was meant to be their destiny since they were the only two lives that survived.
But let’s not forget, being that they are brother and sister and being that there was that global catastrophy, well, who knows what our species might evolve into, depending on the type of catastrophy it is…chemical perhaps?..and the state of the water resources and food supply if there are any left.