The two occasions that Mangan's sister is involved in this narrative both describe her as being surrounded by some kind of angelic aura that clearly shows the narrator's feelings about her and how he thinks that she is some kind of heavenly being--a fitting object of his devotion and affection. Consider the following description:
The light from the lamp opposite our door caught the white curve of her neck, lit up her hair that rested there and, falling, lit up the hand upon the railinlg. It fell over one side of her dress and caught the white border of a petticoat, just visible as she stood at ease.
Mangan's sister is thus depicted as something of a pre-Raphaelite portrait, with a kind of golden, luminous light emerging from her. Of course, what is interesting is that, although she is so important in inspiring the romantic quest of the narrator, she actually plays such a small role in the narrative as a whole. We never are given a description of her face and we don't know her name. Joyce deliberately leaves her a nebulous character to heighten the internal conflict of the narrator between his dreams and illusions and the reality of the world. When, at the end of the story, reality imposes itself when the narrator experiences his epiphany, the boy is forced to realise how he built up an elaborate fantasy over a character who was actually, to a large extent, a complete stranger to him.