It seems like it would be safe to tell us AFTER the pref parties have started. 'Cause once she's at the parties, the sororities will know where she is and where she isn't! So we shouldn't have to wait until after the parties, should we?
If I had to guess, I would say the other is Phi Mu. But whichever group it is, it was her choice to go there, so we'll all be happy!