(CNN) Opening statements in the trial of a New York nanny accused of killing two children she was trusted to care for begin Thursday in Manhattan's Supreme Court.

In a case that shocked families around the world, Yoselyn Ortega stands accused of repeatedly stabbing Lucia, 6, and Leo Krim, 2, to death in their Upper West Side home in Manhattan in October 2012.

It's been over five years since Ortega was charged with the crime and the Krims have waited as Ortega's lawyer, Valerie Van Leer-Greenberg, tried to convince the court that her client was mentally unfit to stand trial.

After several competency hearings, Judge Gregory Carro ruled that Ortega was fit for trial.

A call to Ortega's lawyer was not returned.

Twelve jurors and six alternates have been chosen and are ready to hear testimony in a trial that is expected to last 15 weeks, according to the prosecutor's office.

Ortega faces two counts of first degree murder and two counts of second degree murder.

If convicted, she faces life in prison.

Mother discovers a nightmarish scene

Leo and Lucia Krim

On a typical fall day in late October, Marina Krim, the mother of Lucia and Leo, left her two children in Ortega's care while she took her third child, then 3-year-old Nessie, to a swimming lesson at a nearby YMCA, police said.

Krim had made plans to meet Ortega, who her children affectionately called Josie, at a dance class for Lucia at 5:30 p.m. When Ortega didn't arrive on time Krim grew concerned and returned to her home to check on the children.

When she arrived she discovered the lights were off in the home, which puzzled her. She returned to the lobby and asked the doorman if he had seen her children leave. He hadn't.

"There comes a time when she goes looking for her children and enters the bathroom and finds her 6-year-old daughter and son stabbed to death in the tub," former NYPD Police Commissioner Ray Kelly said.

Krim and her daughter Nessie saw Ortega sitting next to the tub on the floor stabbing herself, in what authorities believe was an attempt to take her own life.

The children's father and Marina's husband, Kevin Krim, was returning home from a West Coast business trip when police reached him at John F. Kennedy International Airport to share the tragic news.

Nanny was considered part of the family

Marina Krim kept an online blog in which she shared photos of her children and her experience growing up with her family. In it, she treated Ortega as a member of the family.

A native of the Dominican Republic, Ortega is a naturalized US citizen. At the time of the deaths, she was living in Manhattan with her son, her sister and her niece, police said.

Ortega was introduced to the Krims through a family friend and had been working with them for two years, police said.

By all accounts, the Krims were happy with Ortega's work.

In February 2012, nine months before the murders, the Krims traveled with Ortega to the Dominican Republic, where they spent nine days, and the Krims stayed at Ortega's sister's home for part of the trip. Marina Krim described the experience as wonderful in the blog.

Grieving family goes forward

Years after the deaths, Marina wrote a post about her experience.

"The weeks that followed were surreal. There were terrifying flashbacks, police inquiries and psychiatrist appointments. There was media attention, an apartment we could never return to, and a memorial service. All of this coupled with the overwhelming grief that always ended in the questions 'How did this happen? Why did this happen?' " Marina wrote.

The how and why still puzzle the Krims, but they haven't let this tragedy hold them back.

"Since Lulu and Leo died, we've had two more children, Felix and Linus. ... They, along with strong Nessie, are genetically and spiritually half Lulu and half Leo," Kevin Krim wrote in a blog post

In memory of their children the Krims created the Lulu & Leo fund in what Kevin called "an act of positive defiance." The fund is a curriculum-based call to action to help children and families foster creative confidence and build resilience.