I got three inbred children in a row with the first generation of inbreeding(half siblings) on my last Zoroastrian play through. One of them was a genius and lived to reproduce as a hilariously screwed but still almost capable first born son. Thankfully his father lived into his eighties so I managed to go elective twice even after I conquered a duchy as a count in his 60's. I got one more on the first generation of inbreeding(full siblings) on the current one.
I've stopped playing them for a while, massive quantities of inbred, fewer lunatics than the Welsh play through I was doing before, me thinks it's broke. It was a bit too much of a challenge, having a third of my offspring be severely retarded, and the same thing occurring with my vassals as I grew larger.
A perfectly run independence revolt on the Abbasids, made successful by me killing 30K+ troops with 6K of my own as the idiot AI wasted their time on sieges, sat at 100% for decades without a surrender being forced in the second play through. It made it all the more irritating to be dodging negative genetic traits on 25% plus of my offspring.