I'm pretty sure that Sumireko. Should we tag it as such?
I agree that it's strongly implied, but in the end: "tag what you see, not what you know". All we have is conjecture based on her being Renko's weird grandma.
If she were at least called by name, that could be a borderline case, but since she isn't and doesn't recognizably look like her (beyond having glasses), I think tagging is too much of a stretch.
