Lets analise this. The old king had a sister that he loved very much, but she ran away to be with a handsome commoner. But then the old king had a daughter who looked like his sister. The old duke married the princess and had Jonas. Going back to the sister, she had a child and that child had Judith and her sister. So Judith's mother and the princess were cousins. Which makes Judith's sister and Jonas cousins as well. So that makes Luca a child born from two people that were quite closely related but they had no idea. And the requirement of succession to winterwald was created so that only the princess's children and grandchildren can inherit it. The old king wanted to prevent the old duke from making a child from another woman his successor.
Luca meets the requirement quite coincidentally from both sides.
Why the marriage? He wants to marry Judith and use the excuse of her royal blood to achieve something like a puppet king. He intends to marry her and say that their child will inherit Winterwald so he will take care of it until the child comes. Obviously he would never hand out the power and even if he thought as far as forcing Judith to have a child with him, the child would most likely be just a shell for him to rule. Doesnt make that much sense actually and its very much wishful thinking that anyone will accept something like that. Hes nuts and thinks he will force everyone to accept him as the next duke if he marries Judith. In short - successor was supposed to be someone from the princess bloodline, which is Luca. Theres no need for inbreeding, the only real requirment is for them to share her blood. But the nutjob is not related at all and its driving him crazy that he will never be a candiadate ┑( ̄Д  ̄)┍
Sooo to succeed Winterwald inbreeding is somehow required? Or did I understand that incorrectly? Why does it require marrying royalty all of a sudden?