>>513189757Every animal has its own sweet spot regarding how close it can breed. For example, rabbits can inbreed to a very high degree before any ill effects while large cats like cougars and lions cannot. The higher genetic variation (diversity) within the group the closer they can breed. Modern humans have a relatively low genetic diversity, which is why most cases of breeding with cousins are disastrous.
If a mutational load is low or nil, even extremely close inbreeding (incest) is safe because there are so few mutations to get double copies of. This is why primordial humans, who had miniscule tribes where everyone was related, were able to thrive despite the fact they were most frequently reproducing with their 1st or 2nd cousins if not their siblings.