To those who speaks about genetic in a world with magic, consider that there is an entire clan that existed for at least more than a hundred years and widely practiced incest with seemingly no bad results. Whishful thinking on the author's part? Irrelevant. That's how the universe work, the reason why works like that doesn't change that you can't assume that they'll have genetic problems when they do it because they know it's positive. They know the positive effects magic-wise, there is no way they wouldn't know the negative effects genetic-wise if they were present.
To those who say that he run away because of his ethics, that's false. He even thought of accepting, but the reason why he didn't is because his prediction that his sister won't have any social life if he does. Given that reason running away like that it's nonsense. You just have to go to your parents and tell them that condition and explain how important it is for her sake. There is no way they wouldn't agree. Additionally as she is head over hills for him, he can try to push her at having more relationships. Hurting her like that it's totally against his objective of doing things for her sake.
I personally can't stand people hurting others "for their sake" while totally ignoring their opposition to it and without any discussion. And as always for those people he did more damage than the one he wanted to avoid. So I'll drop this.