"EN" is meant to be a pun.
It references both the word "energy" as well as en (円), which means "yen".Miriam is Rumi's twin sister.
Both have blonde hair, both have ahoge, both are Muggles who can only provide help from town, and their builds in their portraits look quite similar. It's possible this is common knowledge within the town, including with Erina, and no one just bothers to mention it.