"We're taking it as a suicide, not a drowning," the Los Angeles County Coroner told Us Weekly on Tuesday. Motes was found dead in her bathtub on Sunday.
A source told the New York Daily News that an EMT found a lengthy suicide note at the scene.
"There were three pages of rantings that her sister drove her to do this. There was one page to her mom, telling her how much she loved her, and one page apologizing to [her fiance]" the source said.
Roberts, 46, did not show up to the annual Oscar nominees luncheon on Monday and has canceled her upcoming appearance on Jimmy Kimmel Live.
Motes is the daughter of Roberts' mother and stepfather Michael Motes, though Nancy has described her relationship with her sister as a "work in progress."
“When I was in high school and she was an adult, she would just let me know that I was definitely overweight,” the 37-year-old told The New York Daily News last year. “She would make it quite clear to me and in a not-so-nice a manner.”
Motes said she had high hopes for their future, but tweeted (and deleted) disparaging comments about Roberts just a few weeks ago.
“So my ‘sister’ said that with all her friends & fans she doesn’t need anymore love. Just so you all know ‘America’s Sweetheart’ is a B***H!!’” Motes wrote. “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?”
Roberts helped Motes find work as a production assistant on Glee, where she had been working when she died. Motes had planned a May wedding to fiancé John Dilbeck.
"The family is both shocked and devastated," the Roberts family, which includes actors Eric and Emma Roberts, said in a statement.
[New York Daily News, Us Weekly]
Notable deaths of 2014 [PHOTOS]
Soviets call Reagan joke 'dangerous' [ARCHIVE]