I kind of think two consenting adults should be allowed to marry, across the board, with no arguments.

The reason that brothers and sisters aren't encouraged to marry is that the gene pool [some science-shit here] handicapped babies. I'm personally uncomfortable about people knowingly breeding when their offspring are likely to be handicapped*, but marriage is officially no longer about having children, so I don't see why the having of children should disqualify them.

[sub]* but then, I'm uncomfortable about anybody having children while there are still kids to adopt, but that's a whole 'nother can of worms[/sub]