im with laurelin on this one.
the reason incest is a huge nono in most customs is not because 'its icky' but because of the gentic problems that arise in the offspring!
way back when incest was common, though often not close as brother sister uncle to neice ect was not unusual! they thought it "strengthend" bloodlines kept them "pure" (like linebreeding in dogs) medical science realized this not so muhc the case in humans, especially the closer blood relationships and soon afte rlaws were set in place to prevent the problems, and soon after that...what better way to encourage people to follow those laws than to make incestuous relationships socially unacceptable, this follows throught he generations and in this day and age there is no question incst is bad...but most people never put thought into WHY, just that its "not the done thing"
now personally i have a brother, i grew up with him...could i be in a relationship other than brother sister with him? NO, why? because our relationship dynamic has been strongly structured over the years and i could never see him in a sexual way (for many reasons ontop of "hes my brother")
lets change the person, lets say i grew up with a step brother, who was extreemly hot, not related in any way shape or form other than marriage...
could i have a sexual relationship with him? probably not, again growing up our relationship dynamic would have been set in such a way that he would be my brother, even if not by blood, and while some have a certain moral flexibility, for me, i just couldnt do that.
however if you dont grow up wiht that person, neve rknew that person was your brother/sister, the relationship dynamic changes, you meet you click and possibly fall inlove. for me at least im not sure i could pass up a potential soul mate on the knowledge our dna is the same on one side, if the origional dynamic was ot set as brother/sister before hand.
i mean lis was mentioned, you marry a guy you dated for a good couple years, been married for practicaly foever and find out when your say 80 that you 2 were 1/2 brother/sister al along...then what? get a divnorce after so many years of happy marrigae on the basis that now hes my brither its icky!?
on that note though, i do feel it is extreemly important that these folks should they know and decide to stay together ADOPT or use a sperm donor for children instead simply because of the mental, physical and emothional risks that could have on the child.
the gentic side of things would be too dangerous for me to ever even consider that risk.
but in the hypothetical 80yr old couples case, if they dont know unfortunatly you cant fault them on having children of their own. and once the damage is done...