I very strongly feel like Miller shot the movie as if that was a given. I don't think it was meant to be a reveal as far as he was concerned.

I think Cameron or the studio came along afterwards and tried to retcon it into a twist largely by the editing.

I think the reason they had to do something like that was because they cut (and reshot) chunks of the movie, I suspect to try to prevent anyone reading Grace and Dani as a romance. All that stuff about Dani raising Grace comes from voiceover, and the cheap looking scene of Dani finding her as a kid is a reshoot pasted over a far better scene that links into Grace’s death scene later.

(For the record I really like it anyway.)