MARTINEZ — A brother and sister went on trial Monday on charges they shot to death a 40-year-old Antioch woman in front of her teenage son in 2008.

Richard Terry Gaines, 59, was in the process of moving out of the apartment of his estranged girlfriend, Sheila Hegler. With "building rage" toward the woman and her son, Richard Gaines enlisted his sister, Mary Lee Gaines, 54, of Pittsburg, "to finish what he couldn't," prosecutor Derek Butts said in his opening statement.

On June 18, 2008, the day after Hegler's 18-year-old son, Richard Tucker, and Richard Gaines got into a fist fight, the Gaines siblings returned to Hegler's apartment with Mary Gaines' two sons in tow.

Tucker was standing a foot from his mother when Mary Gaines, a former security guard, pulled out a handgun and shot Hegler in the chest. Richard Gaines then lunged at the gun and ordered his sister to shoot Tony, too, Butts said.

Hagler's son escaped through the front door and was hiding in the parking lot when he heard three more shots fired in the apartment, the prosecutor said. He watched the Gaines drive away before he returned to the apartment to find his mother mortally wounded, Butts said.

Police found Richard Gaines hours later, passed out drunk under a parked car on Aster Drive, across the street from where officers were investigating the homicide. Mary Gaines turned herself in to police in Indiana a few days later. Both are charged with murder;

Defense attorneys David Goldstein and Christopher Bowen assert that neither Gaines sibling shot Hegler nor had reason to do so.

Goldstein, who represents Richard Gaines, said much of the prosecution's case relies on the testimony from Tucker, whose story to police has changed several times.