The sister-in-law of accused Gilgo Beach serial killer Rex Heuermann was left in “profound” disbelief over his arrest for allegedly slaying three sex workers and dumping their bodies on Long Island, according to a report.

Dr. Johanna Ellerup, the half-sister of Heuermann’s estranged wife Asa Ellerup, shared her “stunned” reaction to the charges against her brother-in-law in a statement to NBC NBC News, where she also said she’s incredulous over failing to recognize a sinister side to the alleged killer.

“This disbelief alone is profound and altering,” she said.

“I vacillate between desperately wanting my niece and nephew’s life returned to its previous state, intact, without a father accused of being a serial killer to being pleased and secure in knowing that someone is in custody,” Ellerup told the outlet. 

Heuermann, 59, was charged last week with three counts of murder in the killings of Amber Lynn Costello, 27, Melissa Barthelemy, 24, and Megan Waterman, 22 — three of the “Gilgo Four” who were found in the thicket near Gilgo Beach in 2010. 

“My ego has great difficulty processing the idea that I looked Rex in the eye and was unable to discern any murderous intentions,” she said.

Her half-sister, she added, “is going through a uniquely difficult time.” She declined to comment further, NBC reported.

Asa Ellerup filed for divorce from Heuermann — his second wife — on Wednesday after 25 years of marriage.

Police have said that Heuermann’s family was “shocked” and “embarrassed” when he was nabbed outside his Manhattan office and charged in the grisly killings.

Heuermann, an architect, lived with his special-needs stepson and their 26-year-old daughter, Victoria, in a quiet Massapequa Park home where the suspected serial killer grew up.

A law enforcement source told The Post on Friday that the unsuspecting house may be where Heuermann killed the women while his family was away, as prosecutors have said his victims disappeared when his wife and kids were traveling.

Asa Ellerup, who is from Iceland, is not a suspect, police said.

The source said it also would have been easier to remove the bodies in the burlap bags in which they were found from his home rather than from a hotel where he could be seen.

The victims are all believed to be sex workers that Heuermann solicited online, police said.

Heuermann has additionally been named the “prime suspect” in the 2007 death of Maureen Brainard-Barnes, 25. Police have since expanded their investigation nationwide after learning of Heuermann’s ties to Atlantic City, Las Vegas and South Carolina.