I'm pretty sure it depends on whether or not you fucked her at their house when you met, but another user said that the TrueStory cheat code fixes that.
I've tried a few different save files - not sleeping with her got me a separate event in the hot springs.
As for the code, go to your room > computer > notepad > enter "truestory". Best applied at the start of the game for the change in dialogue between Felicity and Emma; from "clone" to "twin".
