short_review said:
I may have realized something
Back in time, during the Dai Matou Enbu, Yukino spoke about her sister who was taken by some cultists and disappeared.
Could it be that girl from Tartaros? We know that most demons were humans after all, they just transformed them.
Those were Zeref cultists, and her sister was taking to be a slave in the Tower Of Heaven project, it is heavly implied her sister is Angel from the Oracion Seis.