You have to take a step back when reading Eastern stories and realize different places in the world have different views on things, incest being one of them. While in NA it is illegal, in many Asian cultures (and other places around the world) it either is not, or as long as familial consent is given it is acceptable. Now that's mostly for 1st cousins or adoptive siblings, as blood related siblings are generally taboo everywhere. To the original author, adoptive siblings ending up in a relationship is probably not at all unheard of or strange. I've heard plenty of tales, from living people, of 1st cousins ending up together and frankly I find that more unnerving than adoptive siblings, but to them both can be perfectly normal. And at least in this case the author is taking the time to acknowledge that the characters involved know such a change in their relationships would alter the family dynamics entirely.
In the end however, if that is something you can not accept that is perfectly acceptable. You are allowed to be bothered by something, but do know as someone whose read the novel this is not going away. If it is too much for you, best stop reading this story then.