FARGO, N.D. -- A judge on Wednesday agreed to move the trial of a Wisconsin teenager accused of killing his sister.

But Judge John Irby refused to remove himself from the case.

Defense attorney Mark Beauchene had asked Irby to remove himself, alleging the judge was biased against Sergei Carlson. He also asked that the trial be moved from Fargo.

The lawyer for a Wisconsin teenager accused of killing his sister said the judge in the case has told lawyers the only sentence he would accept is life in prison with the possibility of parole. The Fargo judge turned down a plea agreement that included a 30-year sentence, WISC-TV reported.

"This is a very serious case and the court just wants to make sure as we do that the defendant gets a fair trial," said Tracy Peters, an assistant prosecutor for Cass County in North Dakota

The 16-year-old Carlson is charged with murder and a deviate sexual act in the death of his sister, Whitney Carlson, 16, of Fargo, last July. He had been living with his father in Sun Prairie, Wis.

Sergei Carlson was adopted from Russia when he was 7 years old and moved with his father to Sun Prairie, Wis. , in 2002. Whitney Carlson lived with her mother in Fargo.

Authorities said Sergei Carlson told police he strangled his sister in her bedroom with his hands, put pillows over her face to muffle her sounds, and then had sexual contact with her.