I actually have a thought of my own about this. We know that Melia, Erin, Alice, and Allen were able to form into the Archetype (We don't know if that was actually the Archetype or just something that has to do with it somehow). My theory is that if that was actually the Archetype, then that means that they are pieces of the Archetype which Arceus turned into, like mentioned in Nymeria's story. Allen and Alice both recognized Erin so that confirms that they are most likely related somehow and Erin and Melia recognized the voice, from where? I have no idea. But hear me out, I think that Erin and Melia misinterpreted the voice to be their mother, I believe that the voice is actually Arceus.