People just saying this is amoral because they were taught by universal ethics and by most of the religions (which is ridiculous in case of Ch, since a lot of bible characters had incest relations).

Like, 100 years ago being engaged with your cousin were absolutely normal.

In terms of genetics and breeding this is wrong because of degeneration.

I see a problem with protected sex realtions between family. Imagine, you are same age siblings having fun and stuff, but you know you wont be able to be a couple because of genetic thing, this can cause a trouble with finding an actual parter. like if you both love each other in that way, you'll both will struggle with creating a family since you're in love with other person. And there is also a problem of jelaousy