Motes was found dead in a bathtub last February.
The coroner determined that the 37-year-old dog walker and Glee production assistant, who had a rocky relationship with her famous sister, consumed "multiple drugs" before getting into the tub, where she drowned.
Motes' body was discovered by fiancé John Dilbeck, who told the New York Daily News last week that he wasn't invited to the family memorial service.
"It's devastating. I found out Sunday that they had the funeral service without me. They did it so there was no possible way I could attend. I believe that with all my heart. I should be allowed to pay my respects. She was the love of my life, and this would have been our wedding month," Dilbeck said.
Motes openly criticized her sister on Twitter just weeks before her death.
"Do you want to be a fan of someone so cruel? She's not even that good of an actress. Happy you totally F***ed with me?" Motes said.
Roberts spoke with the Wall Street Journal after Motes' death, calling it "heartbreaking."
"It's hour by hour some days, but you just keep looking ahead," she said. "It's hour by hour some days, but you just keep looking ahead. You don't want anything bad to happen to anyone, but there are so many tragic, painful, inexplicable things in the world."