Maryam is the coffee shop owner and resident gossip mistress of Little Syria. Even though she doesn't have a huge presence in the novel, if it weren't for her, we wouldn't have a book at all, since she's the one who gives Arbeely the flask that the Jinni springs from. And she thought it just held olive oil—silly Maryam.
Basically, Maryam just adds some local color. She represents the tight-knit community of immigrants that have formed Little Syria. She helps Saleh when he's down on his luck and confides with him about their mutual distrust of the Jinni, saying, "I never liked [Ahmad]" (23.36). The Jinni is an outsider. He makes no efforts to be an insider, and Maryam distrusts him because of it.