(Spoiler Warning)
Okay, so the cult didn't actually exist, got it. So, what did the grandmother see in the barn? An image of a hissing black cat was shown for just a split second which made the poor woman run screaming back to the house at which point a red liquid began leaking under the door. Are we meant to believe that Emma Watson was behind all of this?
She saw in the dark what any stressed, scared person can "see" in the dark when their imagination and fear take over. The cat was in the barn. She went in looking with worries of some horror on her mind, and saw some large demonic possessed cat face like in the sketch drawing she had been shown earlier.
Yes, Emma Watson's character (Angela) was behind all this, she made up everything to destroy her family that she hated. Not orchestrating every event, but behind the lies that grew out of control and the media, police, and panic took over.