@lucy @kura I think the reason there's ambiguity about this is because leaders of Sumerian cities were considered royal consorts of goddesses, typically but not always Inanna. As a result of this politics of the kings came were intertwined with genealogies and several cities had different genealogies for gods to play up some desired symbolic connection. So if you went from city to city there were different or modified stories based on these changes and the influence of the city affected the popularity of the stories. So there are a lot of alternative stories for Inanna, but not Ereshkigal. Symbolically they were obviously complementary as goddesses of the underworld and of heaven so this explains why they were considered sisters. I don't have any backing for believing this but I think that Inanna-worship was just more desirable since she was the goddess of sex/fertility and heaven, compared to being the goddess of the underworld. Also she had a series of temples populated with priestesses where you could make offerings and have sex with Inanna through a priestess. There were Ereshkigal cults later in Sumer's existence but Inanna was always popularly worshiped.

When I was a college student in ye olde 1996 I drew comic books about sumerian mythology and just spent hours at a time in the library reading tablet translations. At one point I was seriously trying to learn enough German to read some translations only in German.