I believe their plan is:
1. The mother marry a men
2. Get the man to impregnate the daughter
3. Somehow transplant the baby into the mother
4. Have a "happy" family with this new little brother
I think it's extremely unnatural for his mind be that broken virtually by himself. Either someone fucked with him so bad when he was young or there must be something else happening here .
If i have to guess then the moment he decided to go on a date with this girl promted an another unknown girl to stop doing something thus stop her from being killed. But by going on that date, he triggered the rapey groomer uncle to finally act upon his prey.
Fun concept. But most of the historical references, social structures, and architecture are wrong. All of these are basically 5 minutes "top 10 things that is crazy about the Roman Empire" youtube video's level of researching.
Paper as the new and revolutional invention in a early renaissance setting is actually a weird choice. Not only that, the author use the most traditional and inefficient way to make them too. In fact they could had used printing-press as it is not only easily replicated but can have unprecedent...
I know that not everybody can't be J.R.R.Tolkien but just copy paste every single detail of the real world is just lazy and uninspired.
She literally said their version of Jesus and Catholic came out 2000 years ago.