The mother of a prostitute believed to have been slain by a Long Island serial killer was fatally stabbed by her “schizophrenic’’ youngest daughter over the weekend, police and a family lawyer said Sunday.

Psychotic suspect Sarra Elizabeth Gilbert, 27, was “hearing voices’’ right before she murdered her mother, Mari Gilbert, 52, said family lawyer Johnny Ray.

Mari’s eldest daughter and Sarra’s sister, Shannan Gilbert, vanished in 2010, and her remains were later discovered on Long Island’s Gilgo Beach. Law-enforcement sources have said they believe that Shannan and a slew of other people whose remains were also found there may have been murdered by a serial killer.

Mari Gilbert — who fought to have her hooker daughter’s death investigated — was killed Saturday afternoon in the living room of Sarra’s apartment in upstate Ellenville about 90 miles northwest of New York City, police and Ray said.

Sarra Gilbert is currently being held without bail in Ulster County jail on charges of second-degree murder and criminal possession of a weapon.

Ray said Sarra had been arrested in February for drowning a puppy in the bathtub in front of her 8-year-old son — and then taking the boy to the forest and threatening to kill him with a knife, he said.

He said Mari Gilbert bailed her daughter out and took custody of her grandson.

The boy was at Mari Gilbert’s home when she received a call from Sarra asking for help, Ray said.

Sarra had called another sister Sherre earlier that morning to complain of “voices” that were telling her to harm their mother, Ray said.

“She shouldn’t have been out,’’ Ray said of Sarra. “The poor woman [Mari Gilbert] went over to help her daughter, but she was stabbed.

“Mari loved her daughter and went over to help her. The girl was ill, she had been hospitalized several times,’’ Ray said.

“She was a single parent doing the best she could,’’ he said of Mari Gilbert. “She was trying to help her daughter . She was juggling a lot.”

Her daughter Shannan was a 24-year-old Craigslist escort from Jersey City, NJ, when she went vanished after fleeing the home of a john in Oak Beach, LI, in May 2010.

A search for her in December, 2011 led police to a stretch of Gilgo Beach, where her remains and 10 those of other people — including three more sex workers — were found.

While Suffolk County police officials refused to officially linked Shannan Gilbert’s death to the Gilgo Beach murders — the initial autopsy said she had drowned — Mari Gilbert retained Ray to push the investigation forward.

“I believe that Shannan was murdered, and I believe she is part of the serial-killer case,” the mom said in 2011. “We are tired of theories, now we need the truth.”

Law-enforcement sources have said they suspect at least some of the deaths are the work of one or more serial killers.

Ray added, “There was no chance Sarra was involved in the murder of her sister” Shannan — “no chance at all.”

Ray’s assistant, Ves Mitey, called Mari’s murder only the “most recent and tragic turn in the ongoing tale of human misery’’ endured by the family.