Didn’t bother reading since I am already going to assume the mistake that was made.
No, Elune and the Winter Queen are not sisters. It’s Eonar that is her (Winter Queen) sister.
Eonar is who empowered Ysera. Eonar is who chose Ysera to become the Green Dragon Aspect. Eonar is also hinted at further by the legendary you get from the Night Fae.