So there's an achievement in CK3 called "A Perfect Circle" for playing a character with exactly 2 each of parents, grandparents, and great-grandparents, essentially meaning you have to be the child of 3 generations of brother-sister inbreeding. At launch it was extremely difficult to get because the Seduction focus made characters cheat so much that the odds of having a child's genetic parents be its legal parents was shockingly low. Like I want to say less than half.