FARGO - A Wisconsin teenager is expected to plead guilty to killing his 16-year-old sister in Fargo. Lawyers and family members are not talking about details of the plea.

Sergei Carlson, 15, has pleaded not guilty to charges of murder and a deviate sexual act in the death of his sister Whitney, a Fargo South student. A court hearing to change his plea is scheduled for Tuesday.

Cass County State's Attorney Birch Burdick announced the hearing in a statement Friday but said his office would not comment further. Defense attorney Mark Beauchene said it would be inappropriate to talk about the hearing.

Family spokesman Mark Bourdon read from a prepared statement.

"We are not making any comments at this time, so we don't compromise the legal process that is in place," Bourdon said. "Court appearances are always difficult for the family, and we have had many discussions regarding the upcoming change of plea hearing."

Authorities said Sergei Carlson told police he strangled his sister in her bedroom with his hands and put pillows over her face to muffle her sounds on the night of July 14, then had sexual contact with her.

Sergei Carlson, who turns 16 on Monday, 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. Sergei, Whitney and two other sisters spent holidays and summers together, alternating between parents.

East Central District Judge John Irby granted a one-month delay earlier this month so Beauchene could review an expert's report. Beauchene said it was a voluminous file, but would not give details.

Irby earlier denied a motion by Beauchene to throw out Sergei Carlson's confession. Beauchene said the teenager should have had his attorney present during the police interview.