"It looks like it was a drug misadventure. Mixing alcohol and pills. I seriously doubt she took her own life," John Barrymore, 60, told the New York Daily News. (Drew, Jessica and John all share the same father, John Drew Barrymore.)
Jessica, who would have turned 48 on Thursday, was discovered unresponsive inside her Toyota Camry early Tuesday. Her car was blocking a driveway in National City, California, just south of San Diego. She was reportedly found with an energy drink between her legs and dozens of white pills on the passenger seat.
John also said the car contained empty vodka bottles and "a couple bags of meth."
"My guess is that it was the booze and pills, like with Heath Ledger," John told the paper, referring to the 28-year-old actor's accidental drug overdose in 2008.
The Medical Examiner has not yet released an official cause of death.
John Barrymore went on to say that his half siblings spent almost no time with one another, having reunited once after the passing of their famous actor/father.
"I don’t blame [Drew] for being terrified of anyone she’s related to," John said. "Her parents were pretty toxic."
In a statement released to PEOPLE, Drew, 39, said of Jessica: "Although I only met her briefly, I wish her and her loved ones as much peace as possible, and I'm so incredibly sorry for their loss."
Jessica Barrymore / Facebook; Inset: Sara De Boer / Startraks