It makes the most sense with Faye because of the hint we were given through Jane and Dawn's conversation. Plus what someone mentioned earlier which was the connection that Cassie had through Faye's dreams. I think it would be really interesting for these two and their dynamics if they found out they were half-sisters.
But at the same time, I like @PsychoticBeach's theory. Although it's far-fetched, it'll be an interesting twist. Cause right now we all assume it's Faye so if it isn't, that'll be fun to watch as well.