Only hint we get from what happened 7 years ago is that panel on page 34 of chapter 17. Looks like Erika went to Aya after school (on Tanabata day?) and cried, probably telling her to "give her back Koto". Aya being Aya, I doubt she agreed and probably said that it was up to Koto, not to her. A distraught Erika then maybe wrote a wish for Aya to disappear.
Why that wish was magically granted is unknown, but the background theme being Tanabata, light from the stars and the summer triangle, we can infer that these 3 girls represent the 3 stars, Altair, Vega and Deneb.
The thing about these stars is that in the sky they appear close due to perspective, but are actually very far from each other.
Vega represents Orihime, Altair represents Hikoboshi. They're separated by the Milky way. But Deneb isn't part of the Tanabata story. It's an "extra". If we say that Aya and Koto are the "star-crossed lovers" does that mean that Erika isn't part of their story and is the reason for their estrangement?
Then comes Kumagaya-kun. It's hinted he got spirited away too. But he's not in any triangle that we know of, now or in the past. His place in the story isn't clear yet.
Something has to happen for the story to move forward. Erika being on the same stage, as an equal, as Aya isn't going to solve the triangle. Even if she confesses, so what? Something has to give and neither of the 3 MCs are ready to give up on their love interest. The only way at this point is a Deus Ex Machina. Time travel , magic, you name it.