From Doris Bargen, The Search for Things Past in the Genji monogatari:
The beginning of the Genji, as established by literary tradition, is indeed deceptively matter-of-fact about events of grave impact. Genji and his stepmother Fujitsubo commit symbolic incest. Since it is said that the Emperor Hsüan Tsung had originally intended Yang Kuei-fei for one of his sons, Genji’s incestuous love is an even closer analogy to the Chinese legend than his parents’ excessive but otherwise legitimate passion. With this form of illicit love, Genji does more than break a moral taboo; he violates a social religious taboo. To the pursuit of a mother figure is added the cuckolding of the father on the one hand and the blasphemous offense against the sun goddess Amaterasu-omikami on the other.
Bargen is very vague whether this is her own understanding or something that can be inferred from the original text. I don't see this offense mentioned in Genji. Amaterasu, the Japanese sun goddess, is mentioned a couple of times in The Tale of Genji, but not where Bargen is pointing us to -- Genji's affair with Fujitsubo, and not in the manner Bargen describes here. So where in the Tale of Genji does it tell us that Genji's affair with Fujitsubo offends Amaterasu?