The one thing I have not seen addressed, by anyone talking about Eonar being the WQ's sister, is the fact that the titans are the only ones (known) of their kind. One titan awoke, then found the rest, and since then they have travelled from world to world seeking other world souls that will eventually become titans. People keep going by the Chronicle and Ysera's origins there, but those who do have to keep in mind that the Chronicles are from a Titan perspective. That matters because there is no mention of other titans outside the pantheon we know, and that are currently 'sealed' away. So the WQ knows the titans are her siblings, but the titans don't, or have never mentioned it?

As much as it makes more sense for Eonar to to be the WQ's sister based on a power scale, lore wise it doesn't make sense for the WQ to have a connection with the titans based on anything we know atm, especially from the titan's own chronicles.

So the questions remain:

Do the titans just think they created Ysera, but Elune was the true influence?
Or did they actually, and the Eternal ones have a connection to the titans via retcon?
Or maybe Ysera is Elune's pet because she guided her spirit to the Shadowlands?

Another thing to note is we don't have enough information to separate an Eternal One from a First One, properly differentiating the two. A First One could very well be an Eternal One, just one of the first. I think that could fit Elune well, with everything we know thus far. Far better than Eonar does.

One thing is for sure, I can't wait to find out.