Some relatives of Robert Durst’s first wife, Kathleen, a former New Hyde Park resident who was last seen nearly 35 years ago, is asking Surrogate’s Court in Manhattan to declare her dead to bring her “dignity” and name her ex-husband as her killer, their attorney says.

But in an answer filed earlier this month to the request, which the family filed in March, Durst’s Manhattan attorneys, David Friedman and Jessica Rosenberg, charge that the family is attempting to “do an end-run around both the criminal and civil justice systems” by getting the court to say their client killed Kathleen Durst.

Friedman and Rosenberg note that Durst, 73, a real estate heir, has never been charged in his former wife’s death and that he has denied being involved. The lawyers claim that Kathleen Durst’s three sisters are only trying to declare her dead now to pursue their $100 million wrongful-death suit against their client. The civil suit was filed last October.

“Petitioner saw an opportunity for mischief — a chance to reopen the date of death and to use this forum as a back-door to litigate the case of Kathleen’s alleged demise,” the attorneys said of the Surrogate’s Court bid.

Durst is imprisoned in Louisiana on unrelated federal firearms and drug charges and is awaiting extradition for a trial in Los Angeles in the 2000 killing of his friend, Susan Berman 55. Authorities said they believe Durst killed Berman to prevent her from revealing that he killed Kathleen.

Durst admitted accidentally shooting and dismembering a Texas neighbor, Morris Black, 71, in 2001, but he was acquitted in that killing, which he said was accidental and an act of self-defense.

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Friedman and Rosenberg, who declined to comment beyond their court filing, note that technically Kathleen Durst was legally considered dead after having been considered missing for more than three years, but that her family wants to fix her date of death at Jan. 31, 1982, the last time she was seen, to link their client to her death.

“The request is outrageous,” the attorneys said. “This court should not allow itself to be a pawn in Petitioner’s pursuit to create ‘evidence’ for another lawsuit.”

Robert Abrams, a Lake Success lawyer representing one of Kathleen Durst’s sisters, Carol Bamonte, in the Surrogate’s Court bid, said he expects a decision by early fall.

“I think the most important issue here is this is going to bring dignity to Kathleen and make it clear that she is declared dead from Jan. 31 and that he killed her,” Abrams said in response to the accusations of Durst’s attorneys. “Then we can pursue any action against Robert Durst in her murder and involved disappearance.”

The former Kathleen McCormack, who was known as Kathie, married Durst in 1973 when she was 19. Her family and investigators have said they think Durst killed her during an argument at their Lake Truesdale cottage in Northern Westchester County.

In the Surrogate’s Court request, Bamonte, who still lives on Long Island, noted that Durst allegedly admitted to all three killings in the 2015 six-part HBO docudrama “The Jinx.” Durst previously told authorities that he and Kathleen, a fourth-year medical student at Albert Einstein College of Medicine in the Bronx, had fought at their home on that night in 1982, but he said she got on a train to New York City alone and that he did not see her after that. Her body has never been recovered.

Durst married real estate broker Deborah Lee Charatan after obtaining a divorce from Kathleen Durst in 1990.