There's a deep history of the occult here.
www.thrillist.com
Hereditary represents the feature directing debut for Ari Aster, who previously directed six short films. Ever since Hereditary premiered at the Sundance Film Festival in January 2018, critics have compared Hereditary to iconic films like Rosemary’s Baby and The Shining and labeled Aster an...
ihorror.com
I'm not sure how much the director knew about the working occult elite (as opposed to general research needed to make the film) since this was his debut feature. Although being in the industry and studying at AFI he must have at least heard rumours? Do they recruit people secret service style??!
In the second link he says that it was his short films and choice of horror genre that helped get the picture financed.
Hereditary does seem to illustrate what life might be like for people in the bloodline families. Perhaps some of 'their' people heard about the script and made sure it got made, or maybe 'they' saw the short films and pushed the direction of the screenplay to be more comprehensive.
I always got the impression from reading VC articles that people on the families knew everything because they were torturing kids and grooming them from birth. Maybe it is more like in the film Society (1989) and some don't go through it until later. Maybe it is like Hereditary and they never know what is going on. I'm not sure what would be worse