It's more of a coming of age story told from the son's perspective than a story about the mother's descent into insanity. I think she was always crazy, largely because of the part where her sister says that she's like this every year. The paranormal elements are his childish naïveté, while the rest is him growing up and seeing his mother for what she really is (as opposed to someone being harassed by a supernatural force) after seeing it in himself first.