"If I felt like someone was doing something abusive or doing harm, I would report that to authorities," she said.
She said it was a "revelation from God" that the teen girl would marry her husband who was in his 40s.
Under further questioning, the woman admitted that she agreed with the fundamentalist group's 2008 policy that banned marriages for people under the age of 18.
"Eighteen is a good age," she said. "I just think that's a good age to be married."
Earlier, under questioning from her lawyer Robert Wickett, she said that she had a dream in which she saw her future husband.
She said she told her father about the dream and that she wanted to marry.
"He said to keep praying and we'll see what happens."
Later, her father, who went to see the group's "prophet" for advice, told her she was to be married to a Bountiful man.
"It happened to be the same person I'd seen in the dream," she said. "I feel my marriage was a revelation from God."
She said that she learned the name of her new husband only 30 minutes before the marriage ceremony.
The witness said she had one child of her own with her husband and lived in a household with 24 children.
She said she is currently taking a college course to become an accountant and help with the businesses in Bountiful.
Bauman has been asked to decide whether Canada's polygamy law is constitutional. The issue was referred to him after Winston Blackmore and James Oler, two Bountiful religious leaders, had polygamy charges against them stayed in 2009.
A woman from the polygamous community of Bountiful, B.C., told a Vancouver courtroom Wednesday she wasn't worried about her husband marrying a 15-year-old girl because she "seemed very responsible."
The 24-year-old mother, who was married at 17, made the comment during her cross-examination at the polygamy trial in B.C. Supreme Court.
She is the second Bountiful woman to testify anonymously under a court order.
A native of Utah, she said she was married to her husband in the United States in 2003 and then brought across the border to Canada to live in the small, rural community of Bountiful.
She said she was her husband's third wife.
The fourth "sister wife," a 15-year-old American citizen, was married in the United States to the same Bountiful man about six months later and was also brought to Canada to live in Bountiful, she told B.C. Supreme Court Chief Justice Robert Bauman.
The witness, a member of the fundamentalist Mormon group known as the Fundamentalist Church of Jesus Christ of Latter Day Saints, said she believes that to join the "celestial kingdom" when she dies she has to engage in plural marriage.
Under cross-examination from Craig Jones, a lawyer for the B.C. Attorney General Ministry, the witness said she didn't report the teen's marriage to authorities.