You can perform many genetic researches on birds and animals (as their life span is short) to get the trend which may be found in human. Many studies were performed in the following manner.
1. Half sib and full sib mating ( cross between brother x sister).
2. Test cross.
3. Back cross ( father x daughter, mother x son cross) etc.
By many such studies, it was clear that offspring of close relatives have very high chance of gathering of bad genetic characters ( as homozygous recessive Gene gatheration ) and so chance of delivering abnormal offspring is more.
IT IS SCIENTIFICALLY WRONG TO MARRY IN CLOSE BLOODS.