In the original timeline they actually both loved each other but due to her being adopted, both refused to acknowledge those feelings and never speak of them to anyone. The happiness she felt for her marriage was at the possibility of having a decent life, and because it was becoming to much for her to remain in the house with him. He ends up making a wish the day before her wedding wishing for her happiness in love and that's what causes her to go back in time as the universe itself insists for her to be happy in love she has to try again. He himself never expected to have happiness, he just wanted to make sure she was at least happy and they both assumed it would never ever be possible given their family status. As for that status, there is a mention at one point that by their own laws, technically she was adopted by their parents and the moment they died she wasn't exactly a family member anymore. They could have tossed her out and no one would have batted an eye. Also, neither one of them have ever actually looked to the other as a sibling, they just try to pretend hard enough in hopes they will one day believe it. As the story progresses it reveals these things in such a way that for myself, it's not incest at all. But I can also understand why some will never see it as anything but.