Jurors deliberated for three days before delivering their verdicts against Rosa Hill, 36, and her mother, 57-year-old Mei Li of Antioch, for the January 2009 incident in which Selma Hill was killed at her home in Dublin and her grandson, Eric Hill, was wounded.
They both face life in prison without the possibility of parole when they are sentenced by Alameda County Superior Court Judge Kenneth Burr on Aug. 29.
Rosa Hill, who was dressed in black pants and a burgundy jacket and wore her long black hair in a ponytail, rested her chin on her hand after the verdicts were read but otherwise showed little emotion. Mei Li looked straight ahead as she listened.
Eric Hill, who was seated in the second row of the courtroom, breathed a sigh of relief and stretched his neck after the verdicts were announced.
In his closing argument in the lengthy case, which began on April 13, prosecutor Casey Bates said Rosa Hill and Li went to Selma Hill's home in the 7700 block of Peppertree Lane, where Eric Hill and the couple's then-2-year-old daughter also lived, as part of a plot to kidnap the daughter.
Rosa Hill and Eric Hill, who later divorced, were separated at the time and he had been awarded sole legal custody and 85 percent physical custody of the daughter, which meant that Rosa Hill only got to see her 15 percent of the time.
Prosecutors said Rosa Hill was killed the morning of Jan. 7, 2009, and Eric Hill was wounded when he was attacked by Rosa Hill and her mother after he returned to the home with the couple's daughter later that day.
Bates said notes later recovered by police showed that Rosa Hill and Li called their plan "Operation Custody" and purchased a cache of weapons worthy of a small army, including guns, stun guns, a sword, a hammer tool, a crossbow, a throat cutter, a knife and a baton.
In addition, Bates said that when police investigated a computer that Rosa Hill and her mother used, they discovered that the two women had conducted Internet searches on how to get away with murder and how to strangle someone. They had also researched deadly substances such as arsenic, cyanide, strychnine, mustard gas and ammonia, he said.
But Hill's lawyer, Bonnie Narby, said Hill went to Selma Hill's home to check on her daughter, and had no plan to kill anyone.
Narby said Rosa Hill was concerned about her daughter, in part because she thought her husband was molesting the girl -- an allegation Hill has denied. Narby said Rosa Hill also was frustrated that the family court system had had awarded her husband the primary custody of their daughter.
Narby said that if Rosa Hill and her mother had planned to commit murder, they wouldn't have left any evidence behind.
Narby said the fact that Selma Hill suffered 23 wounds, including injuries inflicted by a Taser, indicates that Rosa Hill killed the 91-year-old woman in a sudden confrontation rather than as part of a thought-out plan.
Narby said Rosa Hill "snapped" when she tried to talk to Selma Hill about her concerns that day, and that the elderly woman had told her that she would try to stop her from seeing her daughter again.
Li's attorney, Barbara Thomas, admitted that Li came to Selma Hill's house on Jan. 7, 2009, but said Li only went there because her daughter asked for help and arrived after Selma Hill was dead.
Narby said her only comment on today's verdicts is, "This was a tragic situation I wish could have been avoided."
Thomas blamed the family court system.
"The family court mediator and Child Protective Services are mainly responsible for what happened," she said.
Thomas said the family court system "completely failed everyone and destroyed two families with their incompetence and left a 2-year-old girl with a legacy that she'll probably never deal with."