I think, there is something w’re not considering here.
How the Winter Queen knows Ysera is “of her sister”? Did she knee Ysera beforehand? Or did she sense her sister’s touch on Ysera and just assumed she had relation? Or does she have the power to peer into the soul of someone and know their entire history?
Depending on the answer to that, we can consider the options.
If she got to know Ysera beforehand, it probably means that is certainly not Eonar, since they aren’t closely related, since Eonar has been hiding in Elunaria for almost the entire history of Warcraft.
If she only sensed “her sister’s essence” on Ysera or something like that, it could be Eonar, since we all can agree that the power that turned Ysera into an aspect is from Eonar.
If she somehow knew Ysera’s entire life, it becomes unclear.
Eonar makes sense.
Elune makes sense.
To me, makes sense that the titans and eternal ones share the same origin and they are brothers in a way.
The primus reffers to the eternal ones as brothers and sisters, but the Archon has no relation to the dragonflights whatsoever, and Eonar is a titan, so either.
A- titans and eternal ones are related.
B- Elune is an eternal one.
Which brings a problem here to the table, that is, Elune being an eternal one woulf mean she has her own death realm, and since the night warriors and all spirits related to Elune have been mover to Ardenweald, it means there isn’t an Elune plane of death, which could negate the idea that she is an eternal one, or that Eternal Ones are not necessarily tied to death realms.