Responses

From the raws, John was a devil as a baby (a chapter has a adorable devil infant next to an adorable blonde baby. I don't know if he was a replacement for a twin who passed, or if he was created as an heir since Annette was a girl).
Also, it seems he is not totally aware of what he does as a devil. In the raws he appears to be raping her (holding her hands with his tail) as he comes to his human self distraught that he attacked her.
So judging from the raws, it isn't physically incest. He is a devil.
However, she doesn't know that and considers him both her brother and a man that she wants so she is still breaking social taboo.
I still find it hot as fuck and will ride this tragedy-train into the sunset.