How did it get in the attic? Feels a bit like a plot hole to me that the cult was able to do that unnoticed.
The scene involving them at the end recalled Rosemary's Baby, I liked it. Also similarly, had you guessing if it was really happening or all inside someone's head for a bit.
One thing I noticed duhm, they kept forgetting the damn epi-pen everywhere. Sorry but if your kid has allergies and other issues, you're going to make sure you always have it.
If the cult could make Annie levitate into the treehouse, surely they could make the grandmother's body travel too. There's no need trying to explain the supernatural.
Yes, the movie shamelessly stole from Rosemary's Baby. There's a scene in RB where Mia Farrow is in the foreground, and in the background we see an old man sneakily walk past the doorway. In Hereditary there's a scene with the boy in the foreground and we see something in white crawl by the window in his background. Among other such similarities.