© Provided by New York Daily News Kathleen Durst and Robert Durst are seen in happy times. Her family is taking steps to slap him with a wrongful-death civil lawsuit. - Gerald Herbert/AP

The family of millionaire murder suspect Robert Durst’s first wife took the first step Thursday in pursuing a wrongful-death civil suit against the eccentric real estate heir.

Kathleen Durst’s brother, James McCormack, filed a petition in New York Surrogate’s Court asking for authority over his sister’s estate.

Durst, 72, is in federal prison in Louisiana on gun and drug charges and awaiting trial in Los Angeles for the 2000 killing of writer and friend Susan Berman.

“The reason James McCormack wishes to be appointed administrator at this time is to commence a possible wrongful death action against the decedent’s husband, Robert Durst,” the papers state.

Kathleen Durst disappeared Jan. 31, 1982.

Robert Durst called police four days later and told them his wife had taken a New York City bound Metro-North train from the Katonah train station, near the couple’s Westchester home. Durst was suspected but, due to a lack of evidence, was never formally accused or tried.

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Kathleen Durst was declared legally dead in 2001.

“He killed her and we can prove it,” said McCormack’s lawyer, former Manhattan prosecutor Alex Spiro of Brafman & Associates.

The LAPD reopened its Berman investigation earlier this year, bolstered by new evidence in the HBO documentary by Andrew Jarecki, “The Jinx: The Life and Deaths of Robert Durst.”

Durst appears to make a chilling confession in the final episode of the six-part series when he says off-camera — while his mic is still live — “What the hell did I do? Killed them all, of course.”

Prosecutors suspect Durst rubbed out Berman to keep her from talking to cops about the disappearance of his first wife. Durst denies playing any role in her disappearance, but to avoid investigators he high-tailed it to Texas and started dressing like a woman.

The cross-dressing creep was acquitted in 2003 of the grisly murder of his neighbor in Galveston, Texas, despite admitting that he dismembered the man.

Thursday’s filing, 33 years after Kathleen Durst’s disappearance, was also a result of the oddball confession on HBO, according to her family.

“That, sadly, was the closure we’ve been chasing after for years and years,” McCormack told New York Magazine. “After 33 years of hell — really, there’s no other way to describe what our family has been through — we decided it’s time to sue.”