Erika lived in Hitoyoshi and the Nishizumis lived in Kumamato City, and being 50 miles apart it was no small trip. So Erika must've been visiting relatives in Kumamoto City, but perhaps only a few times which is why neither side recalls who the other person in their memories was.

I think I liked the version better where Erika does remember but is too proud to tell Miho and Maho. More bittersweet that way.
In fact, I'm going to just keep interpreting it that way. How many small children ride around in tanks, specifically panzer IV's, and come in a pair of sisters? Erika should be able to figure it out and here is just mentioning Miho without naming her.