Bonnie Clark Halderman, 60, was found at the home of her sister Beverly Clark, who was arrested over the incident.
"It's still under investigation, and we really don't know what the motive was or what caused this to happen," Henderson County Sheriff Ray Nutt said.
He said officers were called to the house in a rural area near Chandler, about 175 miles north of Houston, on Friday.
The two women were the only people in the house at the time. A knife believed to be the murder weapon has been discovered.
Clark, 54, is being held without bail pending a court appearance and does not yet have a lawyer to speak for her.
Koresh was the leader of the Branch Davidian sect, whose confrontation with federal agents led to a stand-off at the cult's ranch outside Waco.
In 1993, the Bureau of Alcohol, Tobacco and Firearms tried to arrest him for allegedly stockpiling weapons and explosives at the compound.
The confrontation that followed lasted for 51 days and ended when the complex caught fire and burned to the ground.
Koresh and nearly 80 of his followers were killed. Around two dozen children were among the victims.
The US government claimed the Davidians committed suicide by starting the fire and shooting themselves but survivors said the blaze was started by tear gas rounds fired into the ranch.
A 10-month independent investigation in 2000 concluded that Koresh was solely to blame.
Halderman wrote a book about her son called Memories Of The Branch Davidians: The Autobiography Of David Koresh's Mother which was published in 2007.