Only literally towards the end of the series, both got into a misunderstanding, and thought they were blood-related (not).
..Majority of the series, they treated each other as a family because of the idiotic and uncomfortable parent situation. They fell in love normally.
If you still insist though, during the misunderstanding, they were very adamant even then to be with each other. So, in their brain, incest?
But no. They're not even blood-relater to begin with.