First of all, let us consider how the novel describes Mangan's sister, and the curiously nebulous and undefined words that are used. If we have a look at her first appearance, it is hard not to be struck by the way that the description is left deliberately vague:

She was waiting for us, her figure defined by the light from the half-opened door. Her brother always teased her before he obeyed and I sttod by the railings looking at her. Her dress swung as she moved her body and the soft rope of her hair tossed from side to side.

Note how the halo of light that is created from the door makes her seem to have some kind of angelic aura, suggesting that she is some kind of superior individual. Of course, note how this reflection indicates the state of mind of the narrator. He is full of his romantic notions that are able to turn the most ordinary and monotonous happening into something of great mystical significance. He imposes his romantic views on the character of Mangan's sister, transforming her into an object of superiority, beauty and desire, when he really doesn't even know her. The way that the above quote fixates on the swinging of her dress and her hair being "tossed from side to side" reinforces this.