Two people from the B.C. polygamist community of Bountiful will be sentenced on August 11, after being found guilty of taking an underage girl to the U.S. to marry.

The former husband and wife, Brandon and Gail Blackmore, were convicted in B.C. Supreme Court in February for taking the 13-year-old across the border for a sexual purpose.

Crown prosecutors are seeking a sentence of 12 to 18 months for Brandon Blackmore and six to 12 months for Emily Gail Blackmore.

The charges date back to 2004, when the girl was taken to Utah and married to Warren Jeffs, the prophet of the Fundamentalist Church of Jesus Christ of Latter-Day Saints, who is now serving a life sentence in Texas.

"There is no case law to fix an appropriate sentence," said special prosecutor Peter Wilson during sentencing submissions on Friday.

Wilson also asked the judge to take into consideration the vulnerability of the 13-year-old victim who is now in her mid-20's.

"The sentence should not only denounce but deter them and other members of their community from engaging in similar practice."

Acted 'on religious grounds'

Brandon Blackmore's lawyer John Gustafson asked for a conditional sentence, telling the court his client is 71-years-old and has health issues.

Gustafson said Brandon Blackmore — who had spent his whole life in the breakaway church — believed he was acting on religious grounds during the offence but now believes marriage isn't appropriate until the age of 18.

A large portion of the evidence presented during the Blackmores' trial was obtained during the U.S. investigation into Jeffs. Records were found locked away in a Texas ranch that were used to prove the girl was married to Jeffs within days of the leader sending instructions to the Blackmores.

A third defendant, James Oler, was found not guilty of transporting a 15-year-old girl to the States to marry. The B.C. government is appealing that decision.

At the time of the crime the Blackmores were both members of the fundamentalist Mormon church in the southeastern B.C. community of Bountiful, where it's common for men to marry multiple women.

With files from The Canadian Press and the CBC's Bob Keating