So ~
Since the bonus chapter where it was said that the nurse is someone's older sister my first thought was that she was Ryo's big sis, because of the "always smiling" thingy. But quickly brushed it off as it seemed too unlikely.
Then this chapter while looking at her character design I had some kind of epiphany! What if Sumire-sensei is Chizuru's older sister? They both have black hair and green eyes (even if in different shades) and, more especially, they both have a visible "dot" on their bodies. Chizuru's being on her neck and Sumire's on her face.
The only "problem" is that this theory crashes a little with my other theory that Hishiro Chizuru is Subject No. 001, something I've been suspecting since her first real chat with Arata. So, or I'm wrong with one of them, wrong with both or, somehow, both are correct (which would mean that the nurse knows about RELife).