Pretty sure there was a cutscene where they referred to the Primus and Denathrius as brothers too, so maybe they're just fellow "Eternal Ones" which would make Elune an "Eternal One" as well. We still need to find out what Eternal Ones are and where they came from, but they seem to be lords of death in the Shadowlands.
And I wish people would stop saying Eonar is Elune, they're absolutely nothing alike in any way, Elune is NOT a nature goddess, she is literally a death goddess. People would know this if they've been paying any attention to any questline or dialog or location tied to Elune in any way. She empowers warriors to fight better, most quests involving Elune are either related to WAR or communing with spirits, her own temple on the Broken Shore is filled with spirits of ancient worshippers (and literal ghost statues). Biggest clue however is when Ysera died, and Elune showed her power to eclipse the sun and directly claim the spirit of Ysera in front of our very eyes (and then dump that spirit on her "sister's" front yard with a note that says "rez plz thx".)
Elune is a goddess of war and spirits and death, and Tyrande is now an embodiment of Elune's power and fury, going all doomslayer in the maw.