So apparently there are 2 Dimitrakopoulou Streets in Athens, and Google Maps only knows about the wrong one. I walked an extra kilometer to find this out, when I discovered that the street number I'm looking for does not, in fact, exist. Now I have to walk back. I'm steeling myself for that one, and resting a bit.
I also have to hope I find my brother on the way, or at least hope that he didn't make the same mistake I did.