I just realize something
I don't if anyone has made this connection before but each of the girls is into her based on all 5 sense
Smell, taste, touch, hearing and look.
Kinda unrelated, but it would be funny if later there are a scene where Satsuki see a real corpse and can just look at it and figure out how they died and solve the case like a real detective.
There's a physical resemblance but Mitsuki wouldn't call an actual relative '' Kanna-san'', so I guess that theory is out. An old flame of her uncle maybe?
No real reason she wouldn't call an aunt by her name if she got used to it.
For that matter, is there any reason why all the assumptions are that she is the uncle's ex instead of current wife/lover? IIRC, there was no mention of the uncle's status so far.