The Long Island doctoral student who was allegedly raped and strangled to death by her obsessed brother-in-law gushed over him as “one of the most wonderful people in the world” in a haunting Facebook post after he married her sister.

“My sister, my person, my partner in chaos, is now married to one of the most wonderful people in the world,” reads Victoria Castle’s message accompanying a photo of her accused killer Joseph Horner sharing a post-nuptials kiss at their wedding venue.

“I love you both forever!” the post, which has taken a dark new meaning following her horrific death, concluded.

Less than three years later, Horner, 27, stands accused of Castle’s brutal murder, sexually assaulting and then strangling her while his wife was away on a bachelorette trip, cops said.

Prosecutors said at his arraignment that Horner was “obsessed” with Castle for more than a decade before the vicious attack, and lusted over the victim for nearly a decade.

The attack allegedly took place in their shared North Massapequa home June 29.

Horner – a music teacher in the Oceanside School District – lived in the upstairs apartment of the multi-family house with his wife, while Castle lived in the ground-floor unit, according to the Nassau County District Attorney’s Office.

He allegedly asked Castle, a PhD student at Stony Brook University, to help him move a piano before attacking her from behind, placing her in a chokehold until she went limp, at which point he stripped her naked and raped her, officials said.

Afterwards, he called 911 and waited for the cops to show up, admitting to detectives that he had committed the heinous act.

He was arrested for homicide at the scene.

Horner was charged with second-degree murder on a felony complaint and is being held without bail. 

“I am absolutely sick to my stomach, who went to college with Victoria, wrote on Instagram. “She was such a beautiful soul and incredibly intelligent. This is a heartbreaking tragedy.”

In another online post, Sierra Catletti, a teacher who knew Horner’s family, expressed shock over the grisly turn of events. 

“I grew up close to his family and am shocked and sickened by what he did to this beautiful girl,” wrote Catletti, who now lives in Texas.

“Never in a million years would I think he’d be capable of something like this,” she said. “But guess you never really know someone.”

On Wednesday, the Oceanside School District said Horner had been placed on indefinite leave pending an investigation.

“Nassau County officials indicated that these charges have no connection to the school district or its students,” the statement said. “I recognize that this news is extremely disturbing, and I want to communicate with you directly.”