On a more positive note, I've been thinking about how to explain "Kay" in a way that makes some sense within the plot:
It started with Abby's mother. At some point, Abby started too. At some point in a future update, Frank then asks Abby where she picked it up. In the course of the conversation, it comes out that Abby's mother also had this tendency, which could possibly be illuminated with a little story about her.
With this story one could also explain the "kay" of the sisters: The two of them picked it up from Abby and used it at some point. We know that Lilli's and Becca's mother still holds a grudge against the long dead Jessica and maybe this "kay" is something she strongly associates with her hated rival. This could then also give the problems between Lilli and her mother more depth.
Meiko, on the other hand, may have simply copied it from Jessica at some point, after all, she was a regular customer and I think a good friend of Meiko's parents.
So basically: Jessica was the "patient zero" of the whole "kay thing".