Nephi and Lyle Jeffs, considered high-ranking members of the secretive group, testified in a Salt Lake City courtroom that their church doctrine bars them from talking about the sect's dealings.
"I feel like I would be breaking confidences if I discuss the workings of the church," Nephi Jeffs said in court.
Federal authorities say group leaders left phone messages for members telling as many as 1,400 children to take days off school and help with a pecan harvest along the Utah-Arizona border in 2012. Labor lawyers want the brothers to answer questions about working conditions on the farm, as well as the structure and leadership of Warren Jeffs' group, known as The Fundamentalist Church of Jesus Christ of Latter-Day Saints, or FLDS.
The two men appeared in court Wednesday in similar dark suits and ties, both wearing wire-rimmed glasses. They answered questions calmly, but consistently said their beliefs preclude them from giving leaders' names, group records or details about church business.
After their testimony, U.S. District Judge David Sam decided their religious beliefs were sincere, a decision that is expected to put many questions by investigators out of bounds. Sam ruled in September that another Jeffs follower didn't have to testify about the group's inner workings. He cited a U.S. Supreme Court decision that the arts-and-crafts chain Hobby Lobby could be exempt on religious grounds from a federal requirement to cover employees' birth control.
Defence attorney Jim Bradshaw said the federal investigation aims to go beyond the pecan farm and into the workings of the FLDS.
"In some ways, it is harassment," Bradshaw said. "It's a process that would not be tolerated for any mainstream religion."
Federal lawyers say in court documents the company that manages the farm, Paragon Contractors, is affiliated with the sect and was investigated for child labour violations in construction work seven years ago. Government attorneys declined to comment Wednesday.
The Jeffs brothers testified they knew of no criminal activities the church might be involved in. Bradshaw said testimony about criminal activities could supersede the religious freedom exemption, but if that comes up the men could take the Fifth Amendment against self-incrimination.
Sam was expected to decide which questions must be answered during a hearing later Wednesday.
Warren Jeffs is serving a life prison sentence in Texas after being convicted in 2011 of sexually assaulting underage girls he considered brides. Members of his sect, a radical offshoot of mainstream Mormonism, believe polygamy brings exaltation in heaven.
Polygamy is a legacy of the early teachings of The Church of Jesus Christ of Latter-day Saints. However, the mainstream church and its 15 million members worldwide abandoned the practice more than century ago.