The questline involving Tyrande and the Night Warrior power has been shown which you can see here.

https://www.youtube.com/watch?v=qpZBMrkvGao

The ritual to try and share the Night Warriors power doesn't seem to work with Tyrande's fury overwhelming her. Then apparently a cutscene happens which somehow involves the Tear of Elune. Afterwards the Winter Queen is clearly shocked saying, "My Sister... After all this time... She did not abandon me."

This confirms the theory that Elune and the Winter Queen are sisters, but they seemed to have had some sort of falling out with Elune somehow abandoning the Winter Queen, at least how she sees it. It also seems we'll actually hear Elune speak for the first time through Tyrande. Afterwards Tyrande seems to have recovered from the Night Warriors power, so we can safely assume she's either no longer the Night Warrior or some lesser version of it that is not fatal.

Granted, we'll still need to see the cutscene to know exactly what happens, but at this point as far as I can see, the identity of the Winter Queen's sister is confirmed.