For all the normal people who just got exposed to Crusader Kings for the first time, it's not quite as deranged as it sounds. Genius is a very powerful and rare trait in the game, and after a few generations of marrying them it's common to end up related to all of the geniuses alive in your area. Since there's a steep prestige penalty for marrying some commoner below your rank, your genius cousin or sister starts to look like a good choice.
The best trick in CK2 was to start your eugenics program on turn 0 by inviting as many geniuses as you could from all around the world to your court and marrying them into your family immediately. You do all of the invite and marriage offers before you unpause the game and start turns ticking over so that other offers are blocked. Within a generation or two you've got a pretty solid genetic head start over everyone else and gifted/strong/genius descendents are common.
The thing is that royalty IRL did end up inbreeding dramatically to keep things in the royal line, so even when roleplaying in CK, it'd make sense to do it.
For all the normal people who just got exposed to Crusader Kings for the first time, it's not quite as deranged as it sounds. Genius is a very powerful and rare trait in the game, and after a few generations of marrying them it's common to end up related to all of the geniuses alive in your area. Since there's a steep prestige penalty for marrying some commoner below your rank, your genius cousin or sister starts to look like a good choice.