Continuity mistake: Irene puts red paint on the kid's nose which disappears when they all turn around. (00:10:00)
Continuity mistake: When Sister Irene's habit is ripped at the back, you can see a bra strap on her left shoulder which disappears when she is seen at the front and when the camera is closer to her back (when she has the pentagram carved into her).
Continuity mistake: When Sister Irene and Father Burke are eating after he was buried alive, Irene's fork keep changing from in her hand to resting on the plate between shots.
Continuity mistake: When Frenchie drops off food in the beginning, the cold cellar door opens on the right, but when he takes the priest and Sister Irene there it opens on the left.
Continuity mistake: Spoilers: At the end we are shown how this film connects into The Conjuring and shown a scene from that film. However the clip in the nun is different to The Conjuring. Ed Warren has an extra line for example.
Answer: The nun was trying to find the relic. The deceased nuns had no actual idea where it was. That's the whole reason Irene was summoned by the Vatican because she had visions about "Mary pointing the way" also the "ghost nuns" more than likely have something to do with her clairvoyance as well.