Revealing mistake: When Sandra Bullock and Nicole Kidman drag her sister's "dead" husband/fiancé into a room, the door hits him on the head and his eyes move.
Revealing mistake: During the exorcism, we see the women grabbing hands to form the circle, but it is the same three pairs of hands repeated several times, giving the illusion that all 12 women did it.
Answer: She is the one from the opening sequence, their "great, great, great" grandmother who started the whole "curse" thing.
Bruce Minnick