...Plus, could it be an indication that they did, at one point, at least attempt to treat him more like their own son..?

Of course, Harry doesn't seem to remember sleeping anywhere but a cupboard, so that sort of nixes that idea...

But I can see some consessions, at first, Petunia maybe facing some conflicting emotions, which she eventually squashed, perhaps at Harry's manifestations of accidental magic... Perhaps when the differences between clever, cute, wicked Harry and her own lazy son began to become more pronounced...