Texas mother Crystal Williams was this week jailed for 99 years for the brutal abuse and starvation death of her 5-year-old stepson two days after Christmas in 2012.

Little Josiah Williams weighed less than 18kg when he died in her care. Cause of death was given as "dehydration in a battered, underweight child".

According to authorities the boy died after what amounted to six months of torture at the hands of Williams; his father, Charleston Williams, and step-grandmother, Gloria Proo. The last two are still awaiting trial.

Before she was sentenced this week, Williams, 31, told the Texas, San Antonio, court Josiah was so small and thin because he was "picky about his food".

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"I even tried compromising with him," she told the jury. "Giving snacks or doughnuts, just so he would eat something."

But the boy's real mother, Carlotta Balleza, blasted this portrayal in a statement to the court.

"A child died in your care," she said. "Dogs in the street are better mothers."

Police were called to the home of Charleston and Crystal Williams on December 27, four years ago. They were told Josiah wasn't breathing.

They said it was immediately clear he was an abuse victim. He had black eyes, was malnourished, had cuts all over his body and deep gashes in his mouth.

A family friend told the court he had tried to intervene weeks before the child's death.

"He appeared to be underweight and had yellowish, jaundiced skin," Lawrence Walker said.

And Medical Examiner Dr Samantha Evans, who did the autopsy, said it was immediately clear the boy was severely underweight.

"The young man was very thin in appearance where you could see lots of bone structures from the outside," she told the court.

"He also had a number of injuries ranging from what appeared to be acute injuries, things that happened very recently, to wounds that were in the process of healing, as well as a number of scars beyond what we typically see in a 5-year-old that comes through our office."

The jury heard widely contrasting stories. They were shown pictures of the boy during happier times, dressed as Woody from Toy Story and smiling. Then they were shown images of his body after his death.

When Crystal Williams was confronted with these pictures, she broke down sobbing. She broke down again on Wednesday when the jury took less than two hours to find her guilty.

In an emotional closing argument, prosecutor Stephanie Boyd had urged jurors to hand down the maximum life sentence.

Her summary had a number of people in the courtroom openly sobbing.

"What you have to think about is not only the defendant's actions, but you also have to think about Josiah.

"He died in a place with people who were supposed to love him with no help whatsoever. He had no idea that his grandmother was looking for him.

"I think long and hard when I ask for a life sentence for somebody. But this case deserves a life sentence. You know why? Because this little boy did not deserve to die alone, thinking that nobody loved him and that he had no friends," Ms Boyd said.

"He died in a place with people who were supposed to love him, with no hope."

Williams must serve at least 30 years before she is eligible for parole. She was also fined $10,000.

During her trial, jurors were shown video footage from within the San Antonio home. In one room, investigators found traces of blood not far from a hole in a closet door. The hole matched the size of Josiah's head, reported KENS 5 News in San Antonio.

The 5-year-old was not the only child under the Williams' care.

Two other children, aged 8 and 2, were removed by US Child Protective Services. They did not appear to have been harmed.

Josiah's real grandmother, Patty Ojeda-Quintero, told the media in 2013 that when the boy was living with her for eight months before his death, the then 4-year-old was thriving. He was taller than others in his preschool class, loved to play outside and watch Spongebob on TV.

Ojeda-Quintero, who has the ashes of her grandson on her mantelpiece, said Josiah and his real mother had been living with her since October 2011.

But her daughter gave her the authority to care for Josiah after she was arrested on a warrant on April 5, related to a probation violation for a previous drug charge.

She was still in jail when her son was killed.

"What sort of man would let anyone allow another person to harm his children," Carlotta Balleza said to Fox San Antonio in a 2013 interview from prison.

The last time Ojeda-Quintero saw her grandson was on June 29, she told San Antonio website mySA, when his father and Crystal Williams arrived to pick up the boy for a weekend visit.

The next day, she got a letter from their attorney saying Charleston Williams was entitled to a month-long visit, and they wouldn't be returning Josiah until July 31.

"That was fine," she said. "It didn't bother me that they wanted to see him.

"I was glad because they really hadn't seen each other in two years."

She asked to talk to Josiah on the phone that day, to explain to him that he wouldn't be coming back to Grandma's house for four weeks.

"I remember he was talking real quiet, like he was being shy. I asked if he knew he wouldn't be coming home for a while and he said he knew," she said, tears forming.

"I said, 'I love you,' he told me, 'I love you too, Grandma,' and that was it. And that was it."

It would be her last conversation with him.