Nancy Motes, 37, was found dead in a bathtub full of water at a Los Angeles residence on Sunday, L.A. County Coroner spokesman Captain John Kades said.
Prescription drugs were found at the residence, which was not Motes's home. Kades said nothing has been ruled out, including an accidental drug overdose or a suicidal overdose. He added that indications of a suicide note had been found at the residence.
An autopsy for Motes was scheduled for Monday, but an actual cause of death may take as long as 8 to 10 weeks to be determined, pending a toxicology report.
Motes is the half sister of Oscar-winner Roberts and her brother, Eric Roberts, also an actor. She was born to their mother.
Julia Roberts, 46, who is currently nominated for a best supporting actress Oscar for her role in "August: Osage County," did not appear at the Oscar nominees luncheon hosted by the Academy of Motion Picture Arts & Sciences on Monday.
Representatives for both Julia Roberts and Eric Roberts did not respond to Reuters for comment.
In a statement to People magazine, the Roberts family said they were "shocked and devastated" by Motes's death.
"It is with deep sadness that the family of Nancy Motes ... confirms that she was found dead in Los Angeles yesterday of an apparent drug overdose," the family said.