Police were called to a home on Carnegie Court in Chifley at 8.20pm on July 5, after reports that a 16-year-old had been stabbed.
Officers were told the boy was stabbed multiple times.
He was treated by paramedics following the incident and taken to hospital in a stable condition.
Following investigations, an 18-year-old woman was arrested at a home on Maddison Circuit, in St Clair at 9.15am yesterday.
She was taken to St Marys Police Station where she was charged with wounding a person with intent to cause grievous bodily harm (accessory after the fact), and concealing a serious indictable offence of another person.
She has been remanded in custody and will appear in Penrith Local Court on Wednesday. 
A teenage boy has already been charged over the alleged attack and remains before the courts.
