Sarra Gilbert's loved ones said she was never the same after her sister, Shannan, an escort, went missing in 2010 and, 19 months later, was discovered dead near Long Island's Gilgo Beach.

"It's harder this year," Gilbert said in 2011, according to Newsday, "because now we know for sure she's gone."

Her sister's death and the grave circumstances surrounding it "took its toll" on her both emotionally and mentally, her lawyer said.

In the years since the incident, she has been diagnosed with schizophrenia - and, her lawyer said, it was the voices in her head that led her to repeatedly stab her own mother with a kitchen knife and then bludgeon her to death with a fire extinguisher, according to court documents.

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"She imaged her mother as a bad god," lawyer John Ray, who has been representing the family since Shannan's death, told the Washington Post. "Voices were telling her what to do."

Gilbert, 27, who has been charged with murder and criminal possession of a weapon, appeared in court yesterday and apologised for fatally stabbing her mother.

"I love my family, and I'm very sorry for what I've done," Gilbert said in court, according to the New York Post.

The judge ordered Gilbert to undergo a psychological evaluation to determine whether she is fit to stand trial.

In the days leading to 52-year-old Mari Gilbert's death, she had been "dutifully" stopping by her daughter's apartment to check on her because her daughter had been complaining about hearing voices, the family's lawyer said.

When she stopped by on Sunday, Sarra Gilbert allegedly stabbed her mother numerous times with a 38cm non-serrated kitchen knife and then struck her with a fire extinguisher in Ellenville, a village in upstate New York, according to a criminal complaint. Police were called to conduct a welfare check. When officers arrived, they found Mari's car in the driveway but no answer the door.

"One of the officers observed through a window what appeared to be a body lying on the floor, he immediately called for the rescue squad to respond," Ellenville police said in a statement. "The officer immediately made entry into the residence, and was observed by Sarra Gilbert in the kitchen area. Ms Gilbert had blood on her clothing, and she was detained by police at the scene."

Gilbert told the judge that she wanted to forego the psychiatric exam and plead guilty, her lawyer said.

"Sarra wants me to express to the court that she does not wish to submit to a psychiatric exam," Ray said, according to Newsday. "She expressed that she wanted to enter a guilty plea, but this is often the case with people who are not mentally competent, so that expression is not given very much weight by anyone."

But the judge explained to her that she could not plead guilty to a felony in village court, according to the newspaper.

Ray told the Post that he hopes his client is found unfit to stand trial and can get the medical treatment she needs.

"She has a knowledge of what she has done, but she doesn't have an appreciation for what she has done," Ray said. "She knows the facts, but her comprehension of the facts and processing of the facts are awry."

Earlier this year, Gilbert was charged with aggravated cruelty to animals and endangering the welfare of a child after police said she drowned a puppy in a bathtub in the presence of her young son. The case is still pending.

For the past six years, the Gilberts have been caught up in an emotional and exhaustive investigation into the death of Mari Gilbert's oldest daughter, Shannan Gilbert, a 24-year-old escort who vanished in May 2010 after she made a frantic emergency call from a client's home in Oak Beach, New York, screaming, "They're trying to kill me," according to WPIX-TV. A neighbour saw her run from the home towards a swamp behind it.

In December 2011, investigators found her dead in the nearby Oak Beach marsh.

The months-long search for Shannan Gilbert led police to a gruesome discovery - 10 other sets of human remains buried along Gilgo Beach on Long Island's south shore, including bones belonging to a toddler, an Asian man dressed in women's clothes and eight women, several of whom were sex workers, Newsday reported at the time. Police suspected it was the work of a serial killer, but did not believe Shannan's death was related to them.

I love my family, and I'm very sorry for what I've done

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Her family disagreed.

"I'm more frustrated and angrier than ever," Mari Gilbert said following Shannan's inconclusive autopsy report in 2012, according to Newsday.

The Gilberts hired forensic pathologist Michael Baden to conduct an independent examination, which was released earlier this year. The report said there were no signs that the young woman died from drowning, as police had suspected, and noted that she had been the victim of some sort of violence, according to NBC New York.

The family hoped the results would prompt police to take a second look.

At a news conference in February, Sarra Gilbert, who has her deceased sister's name tattooed on her wrist, stood somberly with her mother and her sisters and told reporters, "She is greatly missed," according to WPIX-TV.

Mari Gilbert hired Ray to file a civil suit against a man she thought was responsible for her daughter's death.

"They say she's at rest now, but she's not," Mari Gilbert said at Shannan's funeral, according to NBC New York. "She'll never be at rest until I complete the fight and give her justice."

On a memorial Facebook page set up for Shannan Gilbert, many people have now vowed to continue Mari's fight for justice in the days since one of her daughters, Sherre, confirmed her sudden and shocking death.

"So as a lot of you may know, my mother passed away yesterday. She was murdered by my sister, Sarra who was battling a mental illness for 2 1/2 years," Sherre Gilbert wrote, adding: "My sister Stevie and I are incredibly devastated beyond words as our mother was the backbone of the family.

"Please only offer encouraging words and support while we manage to get through another difficult time in our lives."

Sarra Gilbert is scheduled to appear in court again on August 16.