Elizabeth Edwards, 49, and Katie, 13, were found dead at a house in Dawson Avenue, Spalding, on 15 April.
The boy and girl, who cannot be named for legal reasons, admitted manslaughter at Nottingham Crown Court but pleaded not guilty to murder.
The trial on the murder charges is expected to begin at the same court on 5 October.
The teenagers spoke only to confirm their names and enter their pleas.
More on this and other local stories from across Lincolnshire
The results of a post-mortem examination to establish a cause of death have not been released.
Posting on Facebook at the time of the deaths, Ms Edwards' partner Graham Green wrote: "My babe has gone but you will always be in my heart forever and ever and ever.
"The lady meant the world to me, she was my rock. Katie, so young, lots of good times in front of us been taken away r.i.p."
A friend of Katie's described her as being "like a sister" and Ms Edwards as a second mother.
Two 15-year-olds have admitted killing a mother and daughter but have denied murdering the pair.