I find this all very interesting. I do know that with father to daughter or mother to son breeding the first generation are usually fine but, it is the offspring from the second generation that often goes wrong.
Getting away from horses, there is a family locally that married 'close' first cousin to first cousin twice. The result from the one part of the family I knew would prove that line breeding in humans was not a good thing!
Two sons were certifiable with mental disorders and one of the two daughters exceedingly 'slow' with learning disabilities.
The odds that educated, superiorly intelligent kin with no genetic defects would inbreed is pretty slim I think..