Jessa Duggar Seewald and Jill Duggar Dillard said their brother began inappropriately touching them before they were teenagers.
Dillard said she was 12 and Seewald was "9 or 10," she said.
In the interview, which was recorded Wednesday, the sisters spoke in defense of their brother, who is now 27. He's not a rapist or pedophile, Seewald said.
"I was one of the victims so I can speak out," she told Fox. "In Josh's case, he was a boy, young boy in puberty, and a little too curious about girls and that got him into some trouble," she said. "And he made some bad choices, but really the extent of it was mild inappropriate touching on fully clothed victims. Most of it while girls were sleeping."
"We didn't even know about it until he went and confessed it to our parents," said Dillard, who is now 24. She said what happened to her was "very mild compared to what happened to some."
Added Seewald: "So many girls go through things that are a lot worse."
Sisters say Josh has changed
The Duggars are known for being devout Christians who don't believe in practicing birth control and whose children follow strict courtship rules. The family stars in the TLC series "19 Kids and Counting" The cable network has taken the popular reality show off the air and the future of the program is uncertain.
His parents told Fox on Wednesday that their oldest son molested five girls, four of whom are sisters, more than a decade ago when he was a teenager. Josh's father, Jim Bob Duggar, waited more than a year before notifying police after learning of his son's sexual misconduct.
The parents took disciplinary action and arranged counseling for Josh after he revealed what happened.
Seewald, 22, said her brother came back from counseling a different person.
"I think by the time that ... the report was made and everything, it was very obvious to everyone, us and our family, friends, officials that Josh was a completely changed person," she said. "He had not gone down that path for years. He was humble before God, before us, before the officials that were involved."
They complimented their parents and said they handled Josh's admission and the aftermath well. The parents told each of the girls individually what happened, why it was wrong and explained the changes the family would make to safeguard against any more incidents.
The victims have forgiven Josh Duggar, who was not arrested, the sisters said.
The sisters also told Fox there are no more family secrets.
Magazine obtains police records
The incidents of molestation were a well-kept family secret until two weeks ago, when In Touch Weekly published police records from 2006 obtained through a Freedom of Information Act request.
Juvenile records are usually sealed, and the parents have called the release of the records illegal. Jim Bob Duggar said Wednesday that the family feels targeted by the local police chief, and is considering legal action.
The lawyer for Springdale, Arkansas, who authorized the documents' release, defended his decision this week, saying he acted in "full compliance" with the law.
"The requested record was not sealed or expunged, and at the time the report was filed, the person listed in the report was an adult," Ernest B. Cate said in a statement provided to CNNMoney. "Any names of minors included in the report, as well as pronouns, were redacted from the report by the Springdale Police Department in compliance with Arkansas law prior to release."
But an official for Washington County, where Springdale is located, disputes Cate's contention that it was legal to release the reports because Josh Duggar is no longer a juvenile.
"A juvenile record doesn't cease to be a juvenile record when the person ceases to be a juvenile," Washington County Attorney Steve Zega told CNNMoney on Thursday.
Past weeks have been worse, sisters say
Dillard cried as she talked about the release of the records and the mass of media attention heaped on the family over the past two weeks.
"I see it as a revictimization that's even a thousand times worse" than the molestation, she said. "Because this is something that was already dealt with. We've already forgiven Josh. We've already moved on."
Josh Duggar apologized for his acts in a Facebook post when the record became public, and resigned as head of the lobbying arm of the conservative Family Research Council.
"Twelve years ago, as a young teenager I acted inexcusably for which I am extremely sorry and deeply regret. I hurt others, including my family and close friends," he wrote.
Social media reaction
leaned mostly toward criticizing the Duggar family, though there were a few people who defended the sisters and their kin.
A woman who posts as @jenrobinegan said: "Newsflash: "Improper touching" is molestation. Call it what you want, Josh Duggar is an abuser. #rapeculture"
TV personality Montel Williams chimed in with, "This is like watching Scientology defend itself. Of course these girls are victims #DuggarInterview"
But supporters pointed to the fact that the girls had accepted their brother's apologies.
"Give the Duggars a break. God's grace and forgiveness prevails and is manifested in the restored relationships of the Duggar family," Lydia Talobot wrote.