Lana Wood remembers she was 15 when her sister turned up at the house crying and bleeding.
"I had never seen her that bad. I was asked to leave the room and that something bad happened and that Natalie was leaving R.J," she said. "It wasn't until later that she said she caught him with someone. And that someone was another man."
In 1981, Wood and Wagner were on his yacht Splendour with actor Christopher Walken and Captain Dennis Davern when the actress died.
Her body, clothed in sleepwear, was found in the water a day after Mr Davern says she had an angry argument with Wagner.
Davern has given numerous interviews to media about that tragic night but in an interview with NBC News' David Gregory in 2011 he said, "I made some terrible decisions and mistakes. I did lie on a report several years ago.
"I made mistakes by not telling the honest truth in a police report," Davern added.
He said that he felt Wagner had been responsible for Wood's death.
Walken has rarely spoken about that night but told People magazine in 1986, "I don't know what happened … She slipped and fell in the water. I was in bed then. It was a terrible thing. Look, we're in a conversation I won't have. It's a f***ing bore."
In a 1997 interview with Playboy, Walken, who co-starred with Wood in the 1983 thriller Brainstorm, theorised how she ended up in the water.
"What happened that night only she knows, because she was alone," Walken said. "She had gone to bed before us, and her room was at the back. A dinghy was bouncing against the side of the boat, and I think she went out to move it. There was a ski ramp that was partially in the water. It was slippery — I had walked on it myself. She had told me she couldn't swim; in fact, they had to cut a swimming scene from (Brainstorm). She was probably half asleep, and she was wearing a coat."
But Lana Wood told the host of the Fatal Voyage podcast that she believes her sister was pushed during a "horrible fight" onboard the boat.
"I think it escalated to a point where she was either struck or pushed," she added. "I don't believe that it was planned."
Wagner has co-operated with the police investigation over the years but declined to answer questions when the case was re-opened on the 30th aniversary of Wood's death.
Wagner has not commented on Lana Wood's allegations of a gay love affair.
The series premiered on July 20, the exact day that would have been Natalie Wood's 80th birthday.