Since Zorlun is in the end going to keep scenes with Marc in the game, which means also adding new ones, you can always try to modify the files with the text of the script (but remember you also need to eliminate the related compiled files, or the game will continue to use the original ones).
Only three warnings: 1) Zorlun does not mind if you do that, but do it on your hard disk, in the end, he is doing all the work to write the game, and to create the images, and he needs to live off that; 2) it is something that you have to do at your own risk and trying to think what it means, in the sense that if in doing that something gets broken, there is no guarantee that Zorlun will be able to help you with any error that may appear; 3) over time, depending on the scenes, you may end in situations where it is not so easy anymore to put it as an incest situation, because it is written as "son from a different woman".