Normally, in a first-person narrative, the protagonist is narrating her story. But in "Girl," there's no real narrator. Of course not—there's no action, so there's nothing to narrate.
Okay, so, Girl isn't narrating, but she isn't the one talking either. Or is she? Although the words are all Mom's words, it seems like Girl is recalling all of the advice that her mom gave her as a child and adolescent some time in the future, when she's all grown up. So Girl is the one doing the talking here.
And that is pretty cool. Even though we feel like Girl is powerless at first, because she only "speaks" twice to try to defend herself, Kincaid's narrative technique gives Girl all the power. She chooses the advice that she's telling us, and she puts everything in the order that she wants. Instead of doing all the listening, Girl is doing all the talking. Ah, how the tables have turned!