Anastasia James, 37, smoked the drug before the crash in January 2014 on the M1 in Leicestershire, a jury was told.
Daughter Destiny James-Keeling, 14, and Megan Marchant, 18, died when Mrs James's vehicle came off the motorway at 70mph and struck a tree.
Mrs James denies all charges.
Leicester Crown Court heard Mrs James told police a fault with her Vauxhall Astra convertible must have caused the crash near Shawell.
But prosecutor Michael Evans QC previously told the jury officers found no mechanical defects in the car that could have contributed to the collision.
A blood sample taken from Mrs James showed the presence of the drug's primary ingredient.
Forensic scientist David Berry said traces of cannabis found in Mrs James probably would have "impaired" the motorist.
Mr Berry added it was "impossible" that Mrs James had inhaled cannabis smoke passively.
Mrs James, of Thornton Close, Braunstone, Leicester, told officers she had not smoked the drug on the day of the accident and had last used it "ages ago".
She is charged with two counts of causing death by careless driving when unfit through drugs.
The trial continues.
A mother accused of killing her daughter and a teenager in a crash while under the influence of cannabis blamed the collision on her car, a court has heard.