She clearly doesn't think the relationship is "gross," or that she was ever abused. She was also well over legal age when their relationship was consummated, and they are not biologically related.
The idea of becoming sexually involved with someone you played a hand in raising (even if it was only a minor hand) is kind of weird and vaguely creepy. I absolutely agree with you there. However, given the circumstances at play here, there really isn't anything technically "wrong" with it.
There are plenty of unions out there far creepier than this one that no one seems to have any problem with whatsoever.