Nick Reiner, Jake's younger brother, has been charged with killing the acclaimed Hollywood filmmaker and his photographer-producer wife in their Brentwood, California, home.
He has pleaded not guilty to two counts of first-degree murder and is set to appear in court on Wednesday for a pre-trial hearing.
In an online essay published on Substack, Jake Reiner said he learned of his parents' deaths as he was attending a celebration of life for one of his best friends.
He said he received a call from his sister, Romy Reiner, telling him their father was dead.
"Minutes later, she called back telling me our mother was also dead," Jake wrote.
"My world, as I knew it, had collapsed," he added.
Authorities said the parents were stabbed to death by 32-year-old Nick Reiner, who had a history of mental illness.
"Nothing can prepare you for what it feels like to lose both parents instantly at the same time," Jake wrote. "It's too devastating to comprehend. I still wake up every morning having to convince myself that, no, it's not a dream. This truly is my living nightmare."
Jake, 34, recalled going to see musicals with his mother and attending Dodgers games with his father. He called his father, maker of classic films including When Harry Met Sally and A Few Good Men, his "hero" and his mother "my confidant."
With their deaths, "I was robbed of so many things."
"My parents won't be at my wedding, they won't get to hold their future grandchild, and they won't get to see me have the successful career I'm still seeking," said Jake, an actor. "It simultaneously breaks my heart and enrages me."
He added that he asked for "love and compassion - the same principles my parents lived by."
"Any loss of a parent is devastating, but nothing compares to losing both of them at the same time and, on top of that, having your brother be at the centre of it," Jake Reiner said. "It's almost too impossible to process."
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