How is "Araby" a "coming of age" story, for not only the protagonist but also Mangan's sister?
To be honest, I think it is difficult to argue that this excellent story is a "coming of age" story for Mangan's sister from the text alone. She is a vague, shadowy figure, caught up more in the narrator's romantic illusions of her than anything else. She remains an insubstantial figure, who after she speaks to the narrator and he pledges to go to the bazaar for her, does not appear again. There is no indication that she learns anything or experiences an epiphany, as the narrator clearly does.
The narrator by the end of the story does experience a "coming of age," as we see in the way that his romantic notions of his quest, of the bazaar and of Mangan's sister are, in one moment, rejected as he realises the reality of what is happening to him and who he is. The bazaar is not the magical place that he expects it to be, but a disappointing, shabby affair that is mostly covered in darkness. The final paragraph of this story contains the epiphany that indicates how he develops:
Gazing up in to the darkness I saw myself as a creature driven and derided by vanity; and my eyes burned with anguish and anger.
He realises that his romantic illusions were precisely those: mere illusions which had no impact on reality, and he leaves the bazaar a wiser, if not sadder, individual.