You need to have played with them like 4~7-ish times (might have been changed), to get her to trust you. You'll know when you're able to by talking to her, if her text changes to something about repaying, or whatnot, she 'trusts' you enough now.Anyone else who's experienced being unable to control Sarah no matter what?
Edit: Note, I forgot if you have to get to that point before taking taking Kay's quest, or if it doesn't matter as long as she's at that point when you interact with her.