I'm pretty sure it was stated in the books. I'll look for a reference.
Edit: in WLW, when psatma is arguing with Meppa after the fall of Iothia, she States that Yatwer is the most powerful of all the Hundred. Later, before Esmi confronts Maitha just before he bites it, Esmi is looking at a bunch of idols of the gods, Yatwer among them, and ruminates they are the oldest of the gods. I think there's another reference stating she's the oldest. There was another bit where it was said that even her brother Gilgaol feared her.