The youngest sister of Roberts and actor Eric Roberts, Motes had worked as a production assistant on "Glee" and had been living with her fiance in Malibu. The couple had planned to wed in May.
Roberts's family released a statement Monday confirming 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 statement said. "There is no official report from the Coroner's office yet. The family is both shocked and devastated."
In an interview with the New York Daily News last August, Motes opened up about her estranged relationship with Roberts, revealing that the actress used to tease her for being overweight. Motes underwent gastric bypass surgery in 2010 and lost close to 150 pounds, a transformation she said she hoped would foster reconciliation with her famous half-sibling.
"I think we're both trying to change our opinions of each other," Motes said at the time. "It's a work in progress. It's not going to be fixed overnight, nor do I think it's going to be fixed at this point by me just getting skinny. ... I love Julia absolutely, and I have no doubt she loves me."