Authorities say the boy and his friends were playing with a gun in Baisley Park Pond in Springfield Gardens when it went off just before 1 a.m.
The bullet appears to have ricocheted and struck his sister in the lower back.
She was taken to Long Island Jewish Medical Center in stable condition.
The 14-year-old brother has been arrested and charged with assault and criminal possession of a weapon. A .22 caliber pistol was recovered.
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