SAN DIEGO -- A young woman who allegedly threw boiling oil on her teenage sister after telling her that her beauty wouldn't last pleaded not guilty Monday to a charge of aggravated mayhem.

Amber S. Jackson, 20, was out of custody on bail after pleading guilty to child abduction when she was arrested last Thursday at a San Ysidro motel in connection with the attack on her sister, prosecutor Teresa Santana said.

Jackson allegedly attacked her 15-year-old sister at the girl's home in the 3800 block of 50th Street in Chollas Creek shortly before 2:30 p.m. on Sept. 27, Sgt. Rodney Vandiver said.

Jackson allegedly knocked on her sister's bedroom door, holding the hot cooking oil in a cup, and said, "So you think you're pretty? It won't last," Vandiver said.

She then allegedly threw the liquid at the girl and fled. An ambulance crew took the younger sister to UCSD Medical Center in Hillcrest for treatment of burns to her neck, chest, stomach and hip.

"It's a serious case, and she was out on bail," Santana told reporters. "She harmed her sister."

Jackson has a 2001 robbery "strike" prior as a juvenile and could face up to life in prison if convicted, the prosecutor said outside court.

On Aug. 27, the defendant pleaded guilty to abducting her 5-month-old son from a Chula Vista McDonald's last March 22 during a meeting with a foster mother.

Jackson asked the foster mother if she could take the baby to the bathroom to change his diaper, then disappeared, according to a declaration in support of an arrest warrant.

According to the declaration, Jackson contacted social worker Deborah Jordan on March 25, and left a voice mail message stating she "takes kids away for a living and this time she lost."

Judge David Szumowski set bail at $100,000 on the mayhem case, but ordered Jackson held without bail on her other matter. A preliminary hearing is set for Oct. 18.

Jackson is due back in court Oct. 12 for a status conference.