Rosemary is less affected by Fern’s departure than the rest of her family, because she is too young to truly understand it. Rosemary has no sense of who she is without Fern, and she misses her in a visceral way. She rocks and bites herself until her family intervenes. Now that Fern is gone, Rosemary realizes that her constant chattering is no longer useful or interesting.
Unable to express her feelings about losing Fern, Rosemary turns to nonverbal animalistic behaviors such as rocking and biting herself. Without Fern actually with her, Rosemary behaves more like Fern, metaphorically summoning the presence of her sister even while she is not truly there.