What finally causes Cecile to call Fern "Fern"?

Cecile makes a point of never calling her youngest daughter, Fern, by her name. She wanted to call her Afua, but she was overruled by her husband and Big Ma. From that day on, she's insisted on referring to Fern as "little girl" out of spite, as a way of showing her evident displeasure at the name that she's been given. Fern, however, insists on being called by her proper name, and she gets very annoyed when Cecile calls her "little girl." That's not her real name; it isn't who she really is.
Cecile only calls Fern by her name when she gives the children their early morning wake-up call. This will be the girls' last day with their mother before they return to Brooklyn. Cecile's calling Fern by her proper name foreshadows the final, heart-warming scene at the airport, when Cecile will warmly embrace her children, including the "little girl" Fern.