I know everyone will be saying this within ten minutes, but the HP Lexicon has updated the Black family tree with new information ... and Sirius's parents were second cousins*.

Ah, but purebloods do amuse me.

*Yes, they were second cousins, not third cousins or whatever rubbish other people may be spouting. First cousins have shared grandparents, second cousins have shared great-grandparents, third cousins have shared great-great grandparents, and so on.

Also, it is perhaps interesting to note that as long as you marry as far away as your second cousin (by that measure, the only cousins you can not marry are first cousins and first cousins once-removed) your children shouldn't show the ill-effects of inbreeding because there IS substantial genetic variation. Still, genetic traits will be amplified. So, in other words, there shouldn't be any overt drool.