The reason incest results in deformities & other mental handicap issues is due to;
Almost guaranteed passing on of shit defective DNA helixes (sequencing).
If you have parent A that had no defects in terms of say facial deformities or lung issues & his parents had nothing of the sort then what's being passed along? Parent B did not either but one of her parents had asthma there was a 50/50 split on which parent's DNA would be the dominant one that is passed on to her, she got lucky in the 1/2 chance & received the one that wasn't an issue but if we now factor in legacy DNA there is a 25% chance that their child could be affected by it, genes that skip a generation.
The point is due to the many different genes in play you'll end up with a varied mix with weaknesses & strengths, the lottery coming into play with regards to if you get the good or bad option for each spec tree, some options both being good & others both being bad.
Some get a crap roll while others get a good one yet the potential was about even before the lots were handed out, anyway the problem is if you have a bad trait & the family is inbreeding then that bad trait is 100% guaranteed to be handed down with even more potency than the parents had when you factor in the legacy DNA from 10 generations back & such.
Tell that to the Pakis, highest deformities demographic in uk, also highest cousins marriage rate.