Homicide Squad detectives arrested the 31-year-old woman at Campbelltown Police Station on Tuesday afternoon and charged her with one count of murder and one count of being an accessory after the fact to murder.
She was previously charged with concealing a serious indictable offence in relation to the shooting.
Michael “Mick” Bassal, 44, died after he was shot at the signage business, while his two brothers Mark, 34, and Terry, 41 were taken to hospital with gunshot wounds.
Wayne Williams, 33, who allegedly shot at the brothers, also died at the scene.
Police formed Strike Force Belgica to investigate the incident.
Detectives believe Mr Bassal was shot during a business dispute. The woman was refused bail to appear in Campbelltown Local Court on Thursday.