Hmm...
From Lilly's route:
Lilly (in Northern Sojourn) wrote:Hmm, from memory there's a small town not too far ahead. Other than that though, this is largely just old farmland. Akira and I stayed in our parents' house which was in the nearest city for a while, but after they left we decided to move into a smaller, more easily maintainable, house.
The context in this case is pretty clearly the nearest city to the summer home, and the summer home itself.
And then...
Lilly (in Prelude) wrote:Since I spent quite a few of my vacations here up until I entered Yamaku, I'd say I know it fairly well. We used to drive there once a weekend then. <snip> My family's real house is quite far south. When they left Japan, my parents gave it to Akira and I. Only Akira lives there now, after my moving into Yamaku.
Actually, there's a grammar error in there (Akira and I), but this is... tricky to parse properly. So, the summer home that they visit in Northern Sojourn is a replacement for a larger Satou summer home in the nearest city (I'm going with Hakodate being the nearest city, but the details aren't necessarily important in this case), and not their real home? Or, did I find a continuity error?
And, by "quite far south", how far south are we talking? Does Lilly just not have much sense of scale, and it's quite far south relative to Hokkaido (and may be relatively close to Yamaku), or even just having to go north for a while to get to Yamaku every day? I can't think it's much further south than, oh, Fukushima, given how much evidence points to Yamaku being in Sendai (I know, I know, that's not canon), unless there's also an apartment that Akira and Lilly got in/near Sendai or something.
Edit: Alright, it's south of Yamaku:
Sip (Part 2) wrote:Hisao: "You used to live together? Was it somewhere around here?"
Lilly: "It was pretty far south, so the trip into Yamaku was fairly long. With her working hours getting longer and Yamaku being so far away, there was little choice in the end but to move into the dormitories."
So, relatively close to Sendai (or wherever Yamaku is, anyway), but long enough that it was an arduous drive.
But...
Lilly (in Presents and Presence) wrote:I used to live further north, where there'd be plenty of snow to play in, so it's a little nostalgic. I don't like the heat very much, either.
I think I can build a timeline here - they used to live in Hokkaido (probably Hakodate), moved near Sendai (and by Sendai, I mean "the city that Yamaku's nearest to") for business reasons but kept the Hakodate house for a summer home, and then after the parents left, Lilly and Akira found the smaller house to replace the summer house, but kept the house near Sendai. Is that timeline anywhere near accurate?