This is what the adults in Episode3 say about the homeland. Note that apparently all of them know what is that place and Eva even went there on a vacation.
Spoiler for Episode3:
Eva: ............Odawara is definitely where he was born, but I don't think this hometown Father loved was Odawara. I believe all of us siblings are thinking of the same place."
Rosa: "...Right. This probably isn't Odawara. From what I've heard, he had a very fun time as a youth."
Rudolf: "Hahhaha. ...Father's greatest misfortune in his life was probably being made the Ushiromiya family head."
Krauss: "......Probably. Father really hadn't wanted to become the Ushiromiya family head."
Kyrie: "So where is it? And is there a river where sweetfish swim?"
Eva: "......Maybe they were there at the time. By now, there has been a lot of development there, and the sweetfish might have disappeared. It would be extremely difficult to investigate whether there were sweetfish there during the time Grandfather was a boy."
Krauss: "There is no single river, right? I believe we all investigated thoroughly in a geographical sense. ......You even went to the actual place to investigate directly, right Eva?"
Eva: "That was only as a vacation. ......But the townscape had changed completely from Father's time. After all, it was caught up in the war, wasn't it? It was impossible to even figure out where Father actually lived. ...Probably, even if Father dragged himself to the actual location, by now, even he wouldn't be able to determine where it was that he lived."
Rudolf: "Because that place has had a remarkable revival..."
There is also Shannon in EP1 that says that Kinzo's hometown was probably very far from the Kanto region.