Shocking reason why man 'pushed girlfriend off cliff'

The man accused of pushing his girlfriend off a cliff to her death in 2009 has admitted he was having sex with her underage sister.

Steve Nichols confirmed that he was having a relationship with Rhonda Casto's sister Melanie in an interview with Maria Elena Salinas of Investigative Discovery's The Real Story set to air Monday.

The confession could open Nichols up to statutory rape charges given that at the time he was 34 and Melanie was only 16, reports Daily Mail.

It could also be considered as a motive for Nichols to kill the mother of his then-nine-month-old daughter Ava, whose lifeless body was found on an Oregon hiking trail on May 16, 2009.

Casto's mother Julia Simmons told Salinas: "He told Melanie that she was a dead ringer for her sister, and that the baby would forget her mother within just a matter of a short time and that they could be a family - the three of them."

When asked presented with the idea that Nichols killed one sister to be with the other, he responded: "Well you can believe whatever you want.

"Some people are going to believe I'm innocent and some will believe I'm guilty. You can believe whatever you want."

Rumors of a relationship between Nichols and Casto's sister have been swirling for several years, but this is the first time that he has confirmed them.

Nichols said that he met Melanie about three months after meeting Casto, and that he knew she was underage when he had sex with her.

"It was a very stupid decision," he said.

The now 42-year-old continues to deny playing any role in Casto's death, which is shrouded in mystery and doubt nearly 10 years later.

The Investigative Discovery piece airing Monday at 10pm also revealed that the last words Rhonda spoke to her family included a joke about being murdered, saying: "He is either going to give me a ring or throw me off a cliff, ha ha ha," her mother said.

Right after uttering those haunting words, Rhonda set out on the Eagle Creek Trail along Oregon's Columbia River Gorge, where she died hours later on May 16, 2009.

Nichols told police that the young mom had been skipping down the trail when she slipped and slid into a freezing creek.

He chased after her, but by the time he reached her body he collapsed from exhaustion and hypothermia.

"I fell asleep on her chest and I don't know how long I was asleep for, but I woke up and I realized I was shaking uncontrollably because I was so cold," he told People in 2017.

"I had hypothermia. I made my way back to the car and called 911."

Nichols later alleged that Casto had been high on marijuana and prescription drugs while on the hike.

Simmons has had strong doubts about Nichols' story from the beginning, telling People: "She would never skip down a trail that narrow. She had a baby at home that needed her."

She also said that her daughter had been wanting to get out of the relationship with Nichols for a long time.

"She was going to move out, but she was trying to get up the courage to do it," Simmons said.

"She was worried about not having enough money to support Ava."

Just days after Casto's death, investigators also began questioning the father's account.

Six years later, Nichols was charged with murder in 2015.

In May 2017 he pleaded guilty to criminally negligent homicide and coercion and was sentenced to three years' of probation, with credit for 19 months of jail time.

Nichols alleged that Casto had been high on marijuana and prescription pills when she died. Photo / Facebook

The plea deal for Nichols came after the Oregon Supreme Court ruled in March 2017 that a detective should have stopped interrogating Nichols when the suspect said he did not want to discuss Casto's death.

A Hood River County judge had ruled the February 2015 statement inadmissible, but the state appealed to the higher court, and lost.

Nichols and Casto first became romantically involved in 2005 - when he was 30 and she was 20 - after Simmons introduced the two while renting a room in his three-bedroom condominium in Portland.

"Everything about her was impressive," Nichols said of Casto in 2015. "I just immediately fell in love with her."

Three years later they welcomed Ava, who is now seven.

Nichols told CBS's 48 Hours that the couple took up hiking after Ava's birth because the aspiring model wanted to lose the baby weight.

Simmons said that her daughter told her that Nichols had been trying to convince her to go for a hike for a month before the fatal outing.

One important piece of evidence against Nichols is records that indicate he had increased Casto's life insurance policy to $1million several months before the incident.

Nichols eventually agreed to split the money with Casto's mother and with the estate of his daughter.