MADISON, Wis. -- A Wisconsin man was convicted early Friday of sexually and physically abusing his stepsister, who weighed just 68 pounds and was barefoot when she fled their family's home two winters ago.

Jurors deliberated late into the night before finding the 20-year-old man guilty of first-degree sexual assault of a child, second-degree sexual assault of a child, and child abuse, the Wisconsin State Journal reported. The charges stemmed from incidents that occurred between 2006 and 2011 in the home where the then-children lived with the girl's father and his mother.

The Associated Press is not naming the young man to avoid identifying his stepsister.

After the girl ran away from home in February 2012, she was taken to a hospital where she told investigators her father and stepmother had forced her into their basement, with boarded up windows and an alarm on the door, and refused to feed her. They were convicted on child abuse charges at separate trials.

She also said her stepbrother had sexually assaulted her. Prosecutors said during the trial that the girl was 9 and her stepbrother was 13 when the first of two assaults occurred. They said the man attacked his stepsister because he didn't think he would be caught.

The man pleaded no contest to sexual assault charges in a separate case in November 2011. He also faces sexual assault charges in a case involving a third girl.

The man's defence attorney argued that the girl's story had inconsistences that should prevent jurors from believing the accusations. There was also a lack of physical evidence for the sexual assault charges, as doctors who examined the girl in 2012 said there was no evidence of sexual assault.

But jurors appeared to reject that argument. Sentencing has been scheduled for Wednesday.

The girl had a deep scar on her stomach that she said she obtained when her stepbrother pulled her from a hiding space, where she'd gone to escape the man's beating.

Assistant Dane County District Attorney Matt Moeser told jurors that the girl's father and stepmother put her into in-patient psychiatric care after she told them about the first sexual assault. Her parents later moved her into an unfinished basement and starved her, he said.

Moeser called the living conditions in the family's home the "perfect storm" for the stepbrother's sexual abuse.

Moeser said the parents' punishment of the girl, along with her mistreatment, may have explained her reluctance to tell authorities she had been sexually assaulted in the home.

When she mentioned something in school that led to a social worker to get involved, her parents sent her to live with her grandfather in Minnesota. The girl was later brought back to Madison and home-schooled, though Moeser said she didn't receive much education. She is now 17 years old and still in middle school, he said.

A school social worker became involved after the girl said something at her elementary school that led officials to believe she may have been being abused at home. The girl later told the social worker she wasn't actually sexually abused, a fact defence attorney Ronald Benavides focused on.