^ Keep track of those NPC kids and grandkids. I have a large dynasty on one of my savegames that is rife with inbreeding simply because I did not realize I was marrying one grandkid to another who had defected to another soverign.
I suppose that is one way of punishing wayward progeny... but I think the rack or iron maiden would be a little less creepy than accidental inbreeding.
Also- anyone know what factors decide whether your grandkids and so on stay with you or defect to another sovereign? I've noticed they don't seem as likely to run away if I level them up and give them nice gear- but that might just be because of a small sample size.