Ortega was facing life in prison without parole.
Krim had been at a swimming class with her 3-year-old daughter, Nessie Krim.
"I'm very sorry for everything that happened", Ortega said through an interpreter before the sentencing.
Ortega could be seen stabbing herself in the neck as she stood over the tub. Her defense attorney, Valerie Van Leer-Greenberg, argued she was "suffering from severe psychosis" and "dissociation" at the time of the killings.
The judge calls Ortega "pure evil".
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A NY nanny convicted of fatally stabbing two young children in her care was sentenced on Monday to life in prison without the possibility of parole for a crime that has been described as every parent's nightmare.
Ortega had been working for the family for almost two years when she brought the two siblings to the back bathroom of their Upper West Side apartment and stabbed them to death.
Ahead of hearing her fate, a tearful Ortega, 55, apologized in court to Lucia and Leo's family.
The children were found in a bathtub by their mother when she returned home to the family's luxury apartment building at 75th and Columbus Avenue. Their father was out of town.
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Her lawyer said she suffers from psychosis, depression and heard voices telling her to kill the children.
Ortega is a naturalized USA citizen and a native of the Dominican Republic.
The mother of two children stabbed to death by their nanny at their New York City home says the nanny tried to destroy her family but failed.
But Krim said Ortega failed and instead destroyed her own family.
Ortega had been working for the Krim family for almost two years when she took the children to the back bathroom of their apartment and killed them.
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Yoselyn Ortega, 56, was found guilty in April of two counts of first-degree murder and two counts of second-degree murder for stabbing Lucia Krim, 6, and Leo Krim, 2, to death in their Upper West Side home in Manhattan in October 2012.