Given David's eventual realization that he's in love with Agnes, it's strange how many times he refers to her as a "sister" throughout the novel. It does, however, fit into a broader trend of blurred boundaries between romantic and familial relationships (for instance, Steerforth's relationship with his mother). It also inverts David's dynamic with Dora; whereas David perhaps mistakes longing for his mother for genuine love for Dora, David mistakes his romantic love for Agnes as fraternal affection. This passage also deals heavily with David's incomplete transition to adulthood, and the role Agnes has to play in that; only Agnes, Dickens suggests, can draw out David's latent qualities of perseverance and self-assurance.