How close a relationship will produce an inbred child? What are the consequences of being inbred?
I hope inbreeding won't be based on just being part of the same dynasty as in CK1. If you always married your heir to a daughter of the French king for like 5 generations nothing happened (at least in my games). But if you married someone of a cadet branch of your dynasty, which had been separated for over a hundred years, you instantly got a lisp or hunchback.
By the way, just so everyone knows: a couple cousin-marriages aren't going to screw things up. The bigger problem is when you have first cousins marrying, and their parents are also first cousins, and their grandparents are also first cousins...
Considering the "genetics" system for appearance, it might be more accurate to add some "inbred" genetic traits, where you need to get the "bad copy" from both sides on a few genes simultaneously to feel the effects.