Patricia Houston and Oprah Winfrey Patricia Houston and Oprah Winfrey

Though no official toxicology report has been released stating that the late Whitney Houston died from substance abuse, the singer's sister-in-law and manager says she believed drugs would take part in her demise.

 "The handwriting was kind of on the wall," Patricia Houston says in a special interview with Oprah Winfrey airing Sunday on OWN. Winfrey asked if Patricia thought that drugs would end up taking Whitney's life. "I would be kidding myself to say otherwise," she said. Houston was found dead in her Beverly Hilton hotel room on February 11.

Oprah Winfrey to interview Whitney Houston's daughter, family

Still, Patricia said the pop diva was turning her life around and drugs were less of a worry for the family recently. "Things were really changing with her, very much so. It wasn't about substance abuse or anything like that relative to the latter days... I was afraid for other things."

Winfrey will also interview Whitney's daughter, Bobbi Kristina Brown, who will share her memories of her mother, as well as how she would like her mother to be remembered. The family will also address the rumors surrounding Houston's sudden death at the age of 48.

Check out an excerpt of Winfrey's interview with the Houston family, which will air Sunday at 9/8c, proceeded by a re-airing of Whitney's final TV interview with Winfrey from 2009.

First Look: Patricia Houston on Whitney Houston's Death