>And you know what happens with in-breeding don't you?
In-breeding is bad because it actually decreases genetic variation in a blood line.

That's the long view of why inbreeding is bad, but the short view is more serious.

Humans can and do carry certain "bad" genes that could potentially cause serious defects in their offspring. Many of these defects don't get "triggered" unless both the mother and the father happen to carry the same bad gene. The more closely related you are to your mate, the more likely that your set of "bad" genes will match up with theirs, causing your offspring to have one or more of these serious congenital defects.

Unfortunately, I don't know of any examples off the top of my head of exactly what kinds of defects tend to happen in those cases. Anyone else have some?