Aari-chan
@Meryl: Mmm... good point with the Weasleys.
Maybe the fact there aren't very many purebloods is BECAUSE of the slightly incestuous relationships and marriages they have. I mean, would YOU want to sleep with your cousin? Actually, my thought on the pureblood inbreeding issue is that this is the cause of Squibs in the wizarding society....however I have little canon at this point to back that theory up. :s
Also, Makani did a very funny picture featuring the Black Family Tapestry along those wonderful incesty lines. XD