I don't know why everyone being too serious about incest in the story.
First of all, it's only a story. Of course it will be disgusting in a real life specially if you have sibling in real life.
Second, they are NOT related by blood. One of the most concern thing about incest is that it would danger the child that born from the incest couple, which make no sense if the child birth is supposed to leave descendant.
And, Third the most important thing is... most of modern people seems to forget that marriage means become family. Most of the good marriage I saw is when the couple act like best friend and sibling, so when the passion love is fading away they still want to stay together as family.
Plus, its not like they become sibling since they are born. They become sibling when they are young adult (which is the time when we completely aware of the different of gender and awaken of the sexual attraction), and I said them not only Eugene because Hari is adult mentally.