>What about brothers?
>What about sisters?
>
>What about between siblings (male and female) as long as they don't produce offsprings?
>Why not?
>
>If they say they love each other, who are we to say NO?
Come to think of it, many royal houses did this - either between close cousins, or even between siblings. Including offspring, in such cases.
The idea was, more or less, that the king was so exalted above mere mortals (I just love that hyperlink!) that a marriage with one of them would have been unthinkable.
Not that I agree with this theory, I just wanted to mention it for completeness sake.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)