If you’ve already read the post title, do I need to provide a spoiler warning?
Marnie, who turned out to be Anna’s grandmother, had told her about her life at the mansion and her relationship and life with Kazuhiko during one of Marnie’s bed-ridden bouts with her chronic fevers (note that Marnie’s caretaking duties put a secondary spin on the film title).
Anna wouldn’t be able to recall the specifics of old Marnie’s stories since she was so young and on the verge of delirium—and there wouldn’t really be a story if she knew who the child version of Marnie was, prima facie. Marnie couldn’t have passed on her “story” to her daughter, Emily, since they were separated when Emily was young and had a strained relationship afterward, and Marnie was only 1 year old when Emily died. Anna’s memories of her now-late grandmother and her stories expressed themselves through Anna’s mild narcolepsy and the various micro-traumas of teenage life.
There’s a bit of a curveball in the beginning when we are told through Anna’s internal monologue to expect something magical. But the fact that Anna’s experiences with young Marnie can be attributed to sickness-induced dream expressions, doesn’t necessarily exclude the play of ghosts. Marnie could have worked through Anna’s episodes if she wanted to help spark remembrance. This could be linked to the strong tradition of east Asian culture’s “veneration of the dead” customs and ensuring that the memory of ancestors carry on in their descendants (I believe Anna would be Marnie’s only living descendent).
Also note the inclusion of the Tanabata celebration scene in the film, an event in which two deities are able to meet for a certain very brief time period. This festival is also closely related to the Obon festival, which is celebrated in honor of a family’s ancestors.