Both of Robert Durst’s younger brothers will testify against him at his trial in the murder of best friend Susan Berman, a Los Angeles prosecutor said during opening statements Wednesday.

“You’re going to hear from Douglas Durst, and you’re going to hear from Tommy Durst,” LA Deputy District Attorney John Lewin told jurors.

“They are going to testify at this trial.”

Douglas — chairman of The Durst Organization, a family-owned real estate company — has publicly accused Robert of murdering his wife, Kathie McCormack Durst, who vanished in 1982.

Douglas told The New York Times in 2015 that he believed Robert killed several pet dogs — all Malamutes named “Igor” — as a way of “practicing killing and disposing his wife.”

Douglas said he also suspected Robert wanted to kill him, too, based on a jailhouse recording in which Robert said, “I want to Igor Douglas” — an alleged plot first revealed by The Post in 2003.

“He is a true psychopath, beyond any emotions,” Douglas said.

“That’s why he does things, so he can experience the emotions that other people have vicariously. Because he has absolutely none of his own.”

Robert is on trial for allegedly shooting Berman in 2000 to keep her from cooperating with a renewed investigation into Kathie’s disappearance.

Douglas, the second-oldest of four siblings, edged out first-born Robert to run The Durst Organization, which was founded by their grandfather, Joseph, in 1915.

Douglas supplanted his older brother as heir apparent in 1994 after Robert was caught “peeing in wastepaper baskets” in the company’s offices, he told the Times.

Neither Thomas, the youngest sibling, nor a sister, Wendy Durst, went to work for the family-owned business.

Prosecutors plan to call more than 100 witnesses during Robert’s trial, which is expected to last about five months.