There is only ONE check in the entire game for inbreeding. It is a function called IsSufficientlyRelatedToRuleOutRomance. If you take that one function and cause it to default to false - Poof! you have incest enabled. Nothing else needs to be changed, all the romantic geneaology checks refer back to this one function. Can't get much simpler.
Please don't laugh or just call me stupid for not knowing something. I would like to write mods myself. Would you tell me how to get to the functions you refer to? Please.Thank you for your time.