FARGO - On the day his trial was to start, a teenager accused of killing his sister last summer put his fate in the hands of a judge who told lawyers he wanted a life sentence.

Sergei Carlson, 16, of Sun Prairie, Wis., pleaded guilty Monday to charges of murder and a deviate sexual act in the death of his 16-year-old sister, Whitney, of Fargo. He's set to be sentenced next month.

The Carlsons' mother, Penny Ripplinger, said she was relieved by the agreement.

"This is all very personal to me. Whitney was my baby girl and we were very close. And this is also my son," Ripplinger said Monday, reading from a prepared statement outside the Cass County courthouse.

Authorities said Sergei Carlson told police he strangled his sister in her bedroom with his hands on July 13, 2007, put pillows over her face to muffle her sounds, then had sexual contact with her.

Carlson, who was handcuffed and chained around the waist, rocked sideways in his chair before Judge John Irby asked him Monday to enter the pleas. His only words during the 10-minute hearing were, "Yes, Your Honor."

Defense attorney Mark Beauchene said afterward his client insisted on the last-minute deal because he wanted to spare his family from the trial set to begin Monday in Bismarck. Irby had twice rejected plea deals that called for a sentence of 30 years in prison.

"He was very concerned that this would be very difficult for him and the family," Beauchene said of Carlson. "He was very concerned about having his mother go through the ordeal of having a trial."

Prosecutor Tracy Peters said prison officials told her on Sunday afternoon that Carlson was not interested in going to Bismarck.

"A lot of trials settle at the 11th hour. That's kind of how it happens," Peters said. "This case has certainly taken an unusual path. I think we've been in court many times not knowing what to expect.

"Maybe the family will be able to kind of move on from here and put this part of this case behind them," she said.

Sergei Carlson originally was charged as a juvenile. Because of his age and the severity of the charges, the case was moved to adult court, where the maximum sentence is life in prison.

Irby rejected a plea deal in May that would have called for a sentence of 30 years in prison. The judge said it offered no guarantees that Carlson could be rehabilitated or, if necessary, civilly committed at the end of his prison term. Beauchene wrote in one of the court documents that Irby said he would only accepted a life sentence.

Irby set sentencing for Oct. 23.

Last week, Irby rejected a motion by Beauchene and prosecutors to reconsider the plea agreement. The lawyers presented as evidence a recent mental evaluation saying Carlson could be rehabiliated under the right circumstances.

Beauchene said in an interview Monday that his client has taken responsibility for his actions and his sentence should be consistent with similar cases.

"Of course there can be no guarantees of such rehabilitation. I think that's an unrealistic posture by the court," Beauchene said.

Said Ripplinger, "The last 14 months have been a nightmare, but I have trust in Judge Irby that a proper sentence will be given."

Irby set sentencing for Oct. 23.

Sergei Carlson, who was adopted from Russia when he was 7, had been living in Sun Prairie with his father, the Rev. Scott Carlson. Whitney lived with her mother and sisters in Fargo. The siblings often got together during summers and holidays.

Beauchene said Scott Carlson played a part in Sunday's negotiations, but the lawyer was not specific.

"There were discussions yesterday which led to the ultimate decision, with the assistance of my client's father, to avoid the trial and enter an open plea to each of these counts," Beauchene said.

Mark Bourdon, who has acted as family spokesman, read from a prepared statement he said would serve as the final comments from the family.

"Today we stand before you as a family," Bourdon said. "Our hearts are still broken. Our nightmare has not ended. But one chapter has concluded and this will allow us the opportunity to move forward."