Teenagers admit Spalding manslaughter but will stand trial for murder.
The boy and girl denied murder but pleaded guilty to the manslaughter of 49-year-old Elizabeth Edwards and her daughter Katie, 13.
It is believed the victims may have been killed as early as two days before their bodies were discovered by police at their home in Dawson Avenue, Spalding, on 15 April.
The defendants, who appeared in the dock at Nottingham Crown Court, were 14 at the time of the alleged offences.
They cannot be named for legal reasons.
During a brief hearing, they spoke only to confirm their names and enter pleas of not guilty to murder but guilty to manslaughter.
They are due to stand trial on 5 October.
Both were remanded in custody following their court appearance.