The babies of close (say, brother and sister, or cousin and niece) family members have a higher chance of being born with an abnormality than babies from non-family related men and women. Why?
Many disease/abnormality causing alleles of a genes are maintained in populations through their recessive nature. That is to say, in order for the disease to manifest itself in offspring they must in have TWO copies of the disease causing allele - they must be homozygous for the disease causing allele. Inbreeding leads to a far greater chance of being homozygous at a disease locus if a recent common ancestor of the parents was affected or a carrier. The likelihood of this occurring is measured by the Inbreeding Coefficient.