The appearance of Miina's old friend didn't present any sort of plot twist after all, but it did make for revelations as far as her backstory is concerned. Thus, it was a nice, dramatic episode. And I don't remember hearing "We might be relatives, or we might be strangers" even once! So, no complaints.
The most important piece of story development (or hint of) comes at the very end, however, as Karen discovers the house in the photo couldn't have been the one they originally lived in. Rather, it's the rotting, empty house across the street.
I always knew she was the smart one.
The next installment of the franchise pairs Karen with a gun-slinging, cybernetic android...together, they solve mysteries and fight crime in deepest, darkest Tokyo. Onegai Tanteidan, coming soon!