What Shirley Jackson is trying to convey in Hill House

I'm nearly dine reading Shirley Jackson's The Haunting of Hill House and am still puzzled by the relationship between the central character, Eleanore, and the other young woman who's part of the team staying at HH for a week or so to see if they detect any manifestation of ghosts, Theodora. At times the two women seem to need each other and to get along like sisters or cousins, and at other times they seem hostile and disconnected; I know that relationships can often be that way, on and off, oscillating between extremes, but in a novel that doesn't work as well as in life - it's hard to ground myself into their relationship At times I've even wondered whether the novel is a vast plot against Eleanore, to see if they can trick her into believing there are ghosts and to see if that will detail her fragile personality. At other times, though, I think it's just some inconsistent writing on SJ's part: Some terrific scenes in which the two women huddle together in bed frightened by knocking noises in the hallway, or when they drift away from HH in a walk on the grounds and they oddly experience the entire landscape becoming a bright white light, almost as it it's bleached of all color. (It's typical of SJ to make bright white a color of haunting - she upends so many conventions of the horror genre throughout this narrative.) Toward the end of the novel, the wife of Dr. Montague - who had set up this entire observational experiment - turns up (with her chauffeur in tow!) and expects fine accommodations and tries to take over the mood of the group. She's not, as convention would have it, a skeptic whose sanity is about to be overturned but the opposite - a believer in spirits, but one who approaches them in the most conventional manner: through a ouiji board and through "automatic writing"'; it's as if SJ is saying: All of your presuppositions, dear reader, about the nature of spirits and how they contact with us, are wrong; disbelieve this narrative at your peril.