Hmm tried that, all I got is the sequence where she speaks to you and you can choose to say something about little sister or that you would like that.
Can it be its becasue Im fooling around with Violet??
No.
From checking my old guide which isn't relevant anymore because the game has one built in...
Do the good girl route on the cafe. No slutting it up with the customers, not even showing off a little cleavage. When Kate has trouble with an order, help her out. When you get the walkaround event where Kate takes Chelsea out for kebab, tell her about your parents, and that you'd like her to be her family. When Katie Kaboom calls in sick due to random explosions, buy the most expensive medicine for her, and wake her up. Kiss her on her head to check her temperature. Break Hulk Hogan's legs (important!). When she invites you to a flower festival, go and buy her some damn flowers. She'll ask you out at this point. Say yes.
At some point there will be a conflict with Violet; you'll have to chose. But unless you're doing an all route run where some events are getting overwritten due to too much stuff trying to trigger, you should get the flower festival regardless of whether you're also banging Matt, Violet, or Damien (or Nathan presumably; he wasn't around when I was writing the guide). As long as you're NOT being a slut at the Cafe, period. You can't even do just a little bit of showing off and stop shy of actually whoring yourself out; the game isn't coded for half measures.