I've always assumed jokes about inbred, two headed, Tasmanian congenital idiots were exaggerations. Apparently not. The statistics on inbreeding are quite frightening.
Amongst live births resulting from incest, 6/18 children die in infancy or childhood, and 3/18 have intellectual disabilities.
Here are some statistics on the offspring of first cousins, compared to the children of unrelated parents:
Trait Unrelated 1st cousins
stillbirth and neonatal death* 4% 11%
major congenital malformation 1% 1.7%
genetic abnormality 7.9% 18.8%
mental deficiency 1.3% 5.5%
recessive muscular dystrophy 0.0034% 0.051%
So perhaps it's wise to think twice before taking a plunge into the shallow end of the gene pool. Your sibling may be intelligent and attractive, but chances are your resulting daughter-niece or son-nephew will not be.