We have a curse from the past that lasts for three generations, and which turns out to have a very physical cause, though that cause has an occult origin. There are still questions at the end: how did Mrs. Mothersole get to her final resting place? What exactly did she do?
Another example of MRJ's use of hair as to invoke disgust is found in this story. Here I'm going to quote from my Penguin book again, this time from H. P. Lovecraft in his essay "Supernatural Horror in Literature" (which you can read all of here):
In inventing a new type of ghost, he has departed considerably from the conventional Gothic tradition; for where the older stock ghosts were pale and stately, and apprehended chiefly through the sense of sight, the average James ghost is lean, dwarfish, and hairy -- a sluggish, hellish night -- abomination midway betwixt beast and man -- and usually touched before it is seen. Sometimes the spectre is of still more eccentric composition...
Joshi then takes it further: "[The ghosts] are "lean, dwarfish, and hairy" because they thus embody the primitivism that stands in stark contrast to the learned, rational, skeptical antiquarians who, for James, represented the pinnacle of human achievement."
I'd say that Mrs. Mothersole's witchcraft and the swollen, hairy spiders (ack!) are an example of primitivism, though in this story we don't see much of a learned, rational figure to contrast with her. At least, I don't think of the old superstition of getting advice from the Bible with a pin to be very rational--though it was used here to interesting effect.