In the photos, 5-year-old Nora holds her 6-month-old little sister, Ali.
The two sisters are starting to play together frequently, the zoo said, though Ali still keeps close to their mother, Koola.
The other members of the gorilla family are Koola's other daughter, Kamba; Zachary, Kamba's son; Binti Jua, Koola's mother; and JoJo, a 38-year-old male silverback gorilla.
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