In CK3 you marry relatives because you want your children to be herculean beautiful geniuses

Well that’s clearly not accurate.

to stop your daughters weak claim from becoming a problem.

What does this mean?

In the case of CK we are not talking about inbreeding in the Egyptian dynasties

Obviously. The point is that inbreeding among douchebag families that fancied themselves better than others (Royals) has been extremely common throughout history- Including medieval Europe.

I am not aware of any marriage between siblings among European nobility (between like 600-now),Could you name another one?

Julien and Marguerite de Ravalet they were executed for it though.

Really, First Cousin marriage was only slightly better than sibling marriage anyway considering that a single family (dynasty) would often marry close relatives for hundred of years- it was nearly the same thing when talking about genetic diversity.

For example Charles II of Spain’s family tree was fucked. His parents, grandparents, and great grandparents were all related to each other. His Mom was his Dads Niece, making his Grandma his Aunt, His Maternal Great Grandfather was also his Paternal Great Uncle, and 3 of his 4 of great grandparents were products of incest in the first place. His Coefficient of Inbreeding (COI) was 0.25, which is the same as it would be if his parents were brother and sister.

I’ve thought way too much about incest for one day. Idk if this will help you play your game better or not, but I think this is about as deep of a dive as I can stomach for now.