Dane County Circuit Judge Julie Genovese also gave the 20-year-old man an additional two years for sexually assaulting his then-15-year-old girlfriend in 2011. The sentences will run consecutively.
The sentences come about eight months after Genovese sentenced him to 15 months in prison for sexually assaulting a 14-year-old girlfriend in 2011. He got credit for time served in jail in his stepsister's case and didn't have to spend any time in prison, though.
Assistant District Attorney Matthew Moeser asked the judge to give the man a combined 30 years in prison in the two pending cases, describing him as a conniving sexual predator.
"He has a pattern of sexual offenses that are unrivaled at his age," Moeser said. "He is a dangerous person who seems to have no ability to stop lying, stop manipulating."
The man's attorney, Ronald Benavides, pleaded for leniency. He contended the offenses took place when his client was a teenager, he had been in a relationship with his former girlfriend for seven months, everyone lies and the man grew up in a home with "socially bizarre norms."
The man gave a rambling 40-minute speech to the judge, pleading with her for another chance. He said he'll be punished for the rest of his life because he'll have to register as a sex offender and won't be able to have contact with minors.
"Thank God," his ex-girlfriend's father whispered from the courtroom gallery's front row, where he watched the proceedings along with his daughter.
"I need counseling," the man told Genovese. "I need someone to talk to. I need help ... (30 years) says, 'oh, yeah, you were a victim but we don't care.'"
Genovese acknowledged that the defendant was a teenager when he committed his crimes, adding that most teenage sexual contact isn't prosecuted. His home was marked by "serious dysfunction" and he developed a flawed character from living in it, she added.
But she told the man he still skates through life by lying and the seriousness of his offenses demand prison time.
"You haven't quite gotten that societal rules apply to you," she said.
The Associated Press generally doesn't publish the names of sex crime victims and isn't naming family members to avoid identifying the two girls.
The man's offenses came to light in February 2012 when his stepsister ran away from her family's Madison home. A passing motorist noticed her on the streets, barefoot and wearing only light pajamas. Doctors said the 15-year-old girl weighed just 68 pounds; the motorist who found her said she was so small he mistook her for an 8-year-old.
The girl told investigators that her stepmother and father had kept her in the basement with boarded up windows, motion sensors and an alarm on the basement door for most of the previous five years and didn't give her enough food.
She said her stepmother beat her and her stepbrother tried to have sex with her when she was 9 years old, forced her to perform oral sex on him in 2009 and hit her.
Genovese sentenced the stepmother to five years in prison last summer after the woman pleaded no contest to reckless endangerment and causing mental harm to a child.
The girl's father went through two trials and was ultimately convicted of child neglect, child abuse, reckless endangerment and causing mental harm to a child. He told the judge during his sentencing hearing the girl suffered from severe emotional and behavioral problems that he couldn't handle and that his trucking job kept him away from home and he didn't notice how thin she had become; Genovese gave him five years, too.
The stepbrother pleaded guilty in July to repeated sexual assault of a child in the case involving his former girlfriend. A jury found him guilty this month of first- and second-degree sexual assault of a child and child abuse in his stepsister's case.
"As you can see, my step-brother ... was very rotten to me," the girl, who is now in foster care, wrote in a letter to Genovese this week. "I do not want to see (him) ever again."