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...