I think they confused the names and are talking about having to fuck Zoey also Im pretty sure Fionas sister is lying and was looking at a certain picture on her phone which is why she got so scared and hid her phone or that could have just been put in to throw us off
Pretty sure it was thrown in to throw us off, even if it's obvious it wasn't her for anyone who's paid even a little bit of attention to her. She just doesn't have the personality to do what the blackmailer is doing, and she cares too much about Ashe to actually be outright cruel to her which the 'Duplo Slut' sorta was. Hell, when you arrive at the sleep-over, she's rambling about how important it is that everything goes well in order to impress Ashe to Fiona. How she reacted with her phone, was more than likely just her being awkward and shy around Ashe, like we've already seen that she is on multiple other occasions. Hell, her doing that makes even less sense if you actually try to make out with her in the closet, since at that point Ashe has shown an interest in her which would mean she would have even less reason to try to blackmail Ashe when she then knows Ashe is at least a little interested in her, beyond just seeing her as Fiona's puppy-dog younger sister.
While I'm sure the blackmailer could be someone we already know, frankly it probably is, I also wouldn't be surprised if it's someone we've either not met at all, or just seen in passing at this point. Is that likely? Not really, but I also wouldn't be surprised. Because at this point, we haven't really seen anyone with enough genuine beef to want to do this to Ashe. The only ones I could think would do it, would be that bitch-ass politician or her gold-digger girlfriend. Everyone else Ashe has either known for too long, or haven't at all showed any hints that they would do this sort of thing. But I guess the truth will come out eventually. Guess all we can do is wait and see what happens.