Selene was the sister of Helios, as purely sisterly a being as Artemis was to Apollon. Any marriage between them had to be confined entirely to the invisible, Underworldly regions
Kerényi argues that Selene’s mythological identity is defined by sisterhood with Helios and a strict celestial-chthonic boundary, her famous erotic story originating from Asia Minor and set underground.
, The Gods of the Greeks, 1951thesis