What genetic diseases most commonly occur in a child born from incest (between sibs or between parent & child)?

Varies considerably. The issue is like playing russian roulette. All of us carry recessive genes in our dna that do not express disease because they are balanced by normal genes. Things like cystic fibrosis are carried by 1/20, sickle cell 1/12. All of us carry an estimated 4 defects. Parent child/sib-sib pairings all may have a normal outcome or a bad outcome for any pregnancy.