Nancy, who was the youngest sister of Julia and actor Eric Roberts, was found dead in Los Angeles on Sunday. She had battled with depression and eating problems in the past.
Her loved ones issued a statement on Monday. "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," it read. "There is no official report from the Coroner's office yet. The family is both shocked and devastated."
Julia and Nancy had the same mother, Betty Lou. Nancy's father is Michael Motes, while Julia's is the late Walter Grady Roberts.
August: Osage County actress Julia, 46, was absent from the Academy Awards luncheon in Beverly Hills on Monday, which was attended by fellow Oscar nominees such as Leonardo DiCaprio and Sandra Bullock.
Emma Roberts, the daughter of Eric and niece to both Julia and Nancy, was visibly upset as she stepped out with her fiancé Evan Peters on Monday - her 23rd birthday.
The Nancy Drew actress held on tightly to Evan's hand as they headed out for lunch together in West Hollywood.