Officers were dispatched to the 1600 block of Aster Drive around 6:20 p.m. on a report of shots fired and a female injured. When the officers arrived, they found the victim suffering from at least one gunshot wound.
Police said she was transported to Sutter Delta Memorial Hospital where she died.
Witnesses told police the woman had been arguing with 57-year-old Richard Terry Gaines and 53-year-old Mary Lee Gaines, who is believed to be his sister.
According to witnesses, during the argument Mary Gaines produced a large-caliber handgun and shot the victim.
Police said the victim's teenaged son heard several more shots fired while fleeing the apartment. The suspects were allegedly seen fleeing the scene and are believed to be in Pittsburg.
Richard Gaines is on parole for lewd and lascivious acts with a child under 14 and the California Department of Corrections has issued a parole violation warrant for his arrest.
He is described at a black male, 5 feet 5 inches tall, 140 pounds, with black hair and brown eyes. Police are seeking him for murder, attempted murder, conspiracy to commit murder and being a felon in possession of a firearm.
Mary Gaines is described by police as a black female, 5 feet 3 inches tall, 145 pounds, with black hair and brown eyes. She has been known to go by the names Mary Stokes, Marie Gaines and Mary Dukes.
She is being sought on the charges or murder and conspiracy to commit murder.
Anyone with information about the incident is urged to call Detective Bittner at (925) 779-6939. Anyone who witnesses either of the suspects should immediately call 911 or the Antioch Police Department at (925) 778-2441.