We have this backstory where Donna was Wonder Girl, presumably in the early years of Diana's life in the Man's World. If Diana has never gone back home since she became Wonder Woman, what if Wonder Girl/Donna as the personification of "Truth" was sent to Earth by the Titans of Myth and it was Diana that found her and brought her in as her "ward." Then she was "erased" by Mr. Twister and the twisted version of the Gods took her and made her into something else.
As for Artemis, we do know that there are Rogue Amazons out there that aren't immortal, but are trained. Those Amazons are still out there and Artemis is a part of them. I don't think Diana knows that this version of the Amazons exists.
And Cassie is the daughter of a demi-god in the new reality, so she's actually related to Wonder Woman. But since the Gods WW knows are not the actual Olympian Gods, I don't know how Cassie fits into it.
already knew about Artemis, and there is confirmation in Red Hood and the Outlaws that she does.
It does seem rather confusing on what Diana is aware of and isn't due to the pacing in her own book. If she's doesn't know her own truth, then how can she be running around the rest of the DC Universe so knowing?!?!
Almost seems like their are two Dianas (again).