Rita Bixby, 75, was found guilty of conspiracy and two counts of accessory to murder in the Dec. 8, 2003, shootings.
Ms. Bixby wasn't home at the time of the shootings, but she knew her husband and son planned violence against anyone who stepped on their land, prosecutor Jerry Peace said. The family was angry that the state wanted to take a strip of their Abbeville land for a highway project.
"We're extremely pleased," Mr. Peace said by telephone. "The jury got the evidence, reviewed the evidence, and reached the verdict they thought served justice."
The prosecutor said he thought e-mails Ms. Bixby sent to family and friends in the weeks before the shootings were the strongest evidence.
"I told them if they did anything on our property without our consent, they would be facing a shotgun. I guess you might say two shotguns, which would not be just for show," read an e-mail signed by Ms. Bixby and dated Nov. 4, 2003.
Jurors, who deliberated about two hours, returned their verdict just minutes after the panel asked that the e-mails be reread, Mr. Peace said.
Ms. Bixby's attorney Jeff Bloom, who did not call a witness during the trial, said his client was simply exercising her First Amendment right to free speech in those messages, nothing more.
"It's a tragedy all the way around," said Mr. Bloom, who plans to appeal. "She had never been involved in any crime of violence, not even a parking ticket."
Mr. Bloom said Ms. Bixby was disappointed.
"But she's a strong old lady, and she took the news well," he said. "She's prepared to fight through her appeal."
Ms. Bixby's son, Steven, was sentenced to death in February for shooting sheriff's Sgt. Danny Wilson and state Constable Donnie Ouzts. Ms. Bixby's husband, Arthur, is awaiting trial on two counts of murder.
Sgt. Wilson was the first to die in the shootings that marked the beginning of a 14-hour standoff and shootout with authorities.
He had gone to the home to discuss the Bixbys' anger over the road project, only to be shot while standing on the front porch, his body dragged inside.
Mr. Ouzts arrived to check on Mr. Wilson once radio contact with the officer was lost. He was shot as he stepped out of his patrol car and died on the way to the hospital.