[NorthMountain]

I am given to understand that it usually takes several generations of inbreeding for the really negative genetic effects to start manifesting themselves.

[Harmless Teddy Bear]

Mostly correct. The first generation may or may not have issues. If they do it is usually because both were carrying a recessive gene.

Second generation is where the trouble starts.

With the Egyptians and the Inca Royals where it was usually half brother marrying half sister it often was about the third generation where trouble started and the fourth generation was usually unable to produce viable offspring.