In an agreed statement entered in the Edmonton Court of Queen's Bench, the woman, who cannot be named, admitted to causing the brain injuries and malnourishment that led to the death of M.
The mother also pleaded guilty to aggravated assault in the case of S, M's twin sister, and failure to provide the necessaries of life for both girls.
She also admitted to causing the brain injury that killed M, to subjecting both girls to "significant head trauma" and then lying to police and medical staff about the reasons why. She also admitted to causing the extensive bruising on the girls' bodies.
"The accused admits that her actions ... caused the twins to be grossly malnourished, to suffer ongoing physical abuse, endangered (S's) life, and caused (M's) death."
Last month, the woman's husband was sentenced to 15 years in prison after pleading guilty to manslaughter, aggravated assault and failing to provide the necessaries of life.
Girls weighed the same as 6-month-old infants
The girls were first discovered on May 25, 2012, when their father called paramedics to their Edmonton home because M had stopped breathing.
According the agreed statement of facts, the first responders were "shocked" at the child's appearance, which one paramedic described as "severely emaciated."
"(They) observed her to have very skinny legs and arms, visible ribs, scabs on her skin and both legs, marks on her groin area and bruising to her head."
The little girl had no pulse and wasn't breathing. She was also unconscious. Her sister was "very skinny and unhealthy with a bump on her forehead. (She) had extensive bruising to her face and forehead, appeared sickly, and without any hair."
As paramedics attended to the girls, their older brother came downstairs.
He "began playing, talking in another language, and jumping on the sofa," the court document said. "He appeared to be in good health and did not have any injuries."
The girls each weighed the same as an average six-month old, the court document said. They were nearly 28 months old at the time.
S survived. M remained in hospital until September 2012 when she was taken off life-support in a case that made it to the Supreme Court of Canada.
The father, mother and children cannot be named under Alberta's Child, Youth and Family Enhancement Act.
The mother's next court appearance is scheduled for July 22