Did you finish the quest in day 25? you must talk in the lab then guy in the back of uwus (and thanks for reminding, I need to track that one asap)
Are you sure about the seven deathly sins quest? didn't you get maid oufit? I'm asking because I personally tested it and everything seemed okay by day 18 they all spawn and they are in: Town hall front, Burger shop, love hotel, game store (east zone), old mansion, inside the mall and around the bar. I think the dialog could be bugged tho.
Also, it gives you the chance to get a strong weapon based on preferences.
You can defeat easily the 7 demons with gimmicks.
Greed for example you toss money and he will lose half hp
every sin has something like this.
I will check the hooker/npcs I might have an idea whats happening