Drew Barrymore's Half-Sister Dead -- Jessica Barrymore Found Dead in

Jessica Blyth Barrymore, the half-sister of Drew Barrymore, was found dead in her car early Tuesday morning in San Diego.

Jessica's body was discovered by one of her neighbors, who told 10News ... she had a soft drink between her legs, and there were dozens of white pills scattered on the passenger seat.

Jessica and Drew had the same father -- John Drew Barrymore -- but they were not close. Jessica's mother is actress Nina Wayne ... who was John's 4th wife.

Through a rep, Barrymore tells TMZ, "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 would have turned 48 on Thursday.