Well, to make a long story short, the incest was not intentional or forethought. Without posting a plot spoiler, let me just say it's a tragic story, already in the Silmarillion - and well worth reading. The Professor thought it was worth telling, as it is easily the longest part of the book.
I read the Silmarillion once years ago... I'll have to ask my mother if she still has it.
I always opined that it was pretty much Melkor's doing - a good example of him wasting his gifts on the trivial...