Ohhh brother.... :-(

I honestly don't see what the problem is - I can't believe they would separate after finding out the news, but being an only child I don't have quite the same perspective. Still, though, if you never regarded the other as your sibling, and one day were told he/she was, I find it hard to believe that would destroy the deep connection the two of you must have had to get married. Pretty tragic, really, that the taboo was enough to ruin it for them...
 
Well they could always adopt, and she could get her tubes tied/he a vasectomy if the law wasn't willing to take their word for it.
 
I mean that's true too,
but the whole social issue would make it pretty difficult for them to be a normal family. That kid either way would have it extremely rough. Even if they did really love each other, there's nothing that would be able to change the way they were viewed. That's basically the ultimate taboo.
 
funny, i guess you have to be a king or a queen to sleep with your brther or sister :) all the old royal families had all kidns of interbreeding...which is how we learned it was a bad idea :)