Nancy Motes, 37, was found in February submerged in a bathtub after consuming drugs, said Craig Harvey, chief of coroner's investigations.
The younger sister of the Oscar-winning actress was found in the home by a friend. Investigators found prescription and nonprescription medications at the scene and took them as evidence. Coroner's officials did not identify the drugs in its findings.
In the wake of Motes' death, the star's family issued a statement saying they were "devastated" and Roberts laid low during the surrounding media frenzy.
Harvey said a full version of the coroner's report on Motes is expected to be available in two weeks.