Since in fashion (or rather in the game) there are problems with the fact that the developers are firmly convinced that 50-80% of all children from a brother-sister union will definitely be degenerates (in the second generation of incest, the game-mechanical chance is more than 100%) how about adding mod a cultural tradition that reduces the chance of inheriting negative congenital traits when close relatives marry? Something like the "Kinship through Blood (unrestricted marriage cultural tradition)" mod?