The alleys seem... very badly broken at the moment, or else something is messed up with my installation. The walls aren't solid until you navigate around the pair -- just the blackness -- and the scene you're supposed to get the first time you approach them didn't trigger for me until I tried going through a second time. (And trying a third time, after sleeping, the pair are still there and I got groped by one of the guys again... though maybe that's 'cause I didn't do anything with Ami?)
EDIT: Also something feels like it's up with Mia and Ami. Their names are anagrams, they run perverted and innocent versions of the exact same business, and their facial features and eye colors are similar enough they could pass for twins. Maybe I'm reading too much into it, but if it turned out they were actually twins, or that Ami was Mia while under the effects of some kind of curse, I would be... extremely unsurprised.
EDIT 2: Geez, I hope if I called it, you're not the kind of writer who feels like they have to change plans or twists if someone guesses them in advance.
EDIT 3: ... What is the Sheer Skirt/Blue Detached Sleeve Blouse outfit a reference to? At first I thought Mima, but not only does it not quite line up with her outfit, that's a heck of a deep cut for a random outfit in the game.
EDIT 4: Well, second guess was a dud. The God of Wind reference made me think of Annet Myer and El Viento, but it's an even worse match for that one.
EDIT 5: ... Wait, even as a blind guess, why the heck did I consider Arle Nadja?
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I think I've figured out the cause of the alleyway wall issue. There's actually two maps for the alleyway, one that is just for the intro, and the main alleyway area. The intro, for some reason, didn't have this sort of dirt/smudge effect overtop of it, so I recently corrected that. However, when I put that effect on a tile, it makes it so you can walk through it, so I needed to indicate that you cannot walk through all of the alleyways walls. That's why it works after you get around the pair, because then it puts you on the regular alleyway map. I've uploaded a fix for the collision issue. My best guess for not seeing the first event with the pair is that you avoided the trigger for the event by going on the walls, because as far as I can tell it is functioning at the moment. Patch 0.45.7 should fix this.
As for Mia and Ami, your guess is correct, they are sisters. I plan to have more content for them which would eventually lead to that, but I hadn't gotten around to it yet. I won't be changing it just because you figured it out though lol, part of the point of having all the hints linking the two is so people can figure out that they are related somehow to begin with.
Lastly, the sheer skirt/blue detached sleeve blouse outfit is Aqua's outfit from Konosuba. I used to have a custom outfit tier on Patreon where a patron could request certain clothes to be added to the game, and it's one of those.
Thanks for the bug report, and I'm happy to know someone noticed all the similarities between Ami and Mia, so thanks for sharing your theory on that as well!