I can’t believe my second comment on Lemmy is gonna be about incest.
If you only have great genes, multiple generations of sister-wives will produce children with those exact same great genes. The problem with incest is that if you carry alleles for recessive disorders (and most people do), inbreeding makes it more and more likely that two copies of the recessive gene will be inherited and expressed since family members generally carry the same recessive genes. That’s why banging strangers is generally a good idea, since they usually carry a different set of recessive disorders than you do.
If there were a brother and sister (or any pairing) with a pristine genetic code, then as long as they remained inbred the first birth defect or genetic disorder to affect their family line will be a completely novel random mutation that formed as a result of pure time and chance over dozens or hundreds of generations. It’s also why inbreeding is a standard tool for animal and plant husbandry.