Sidney-Splaver died Monday of kidney failure at Swedish Medical Center in Seattle, said her husband, Rick Splaver.
Born April 16, 1935, in Los Angeles, she was raised on movie lore and the entertainment business. She was the daughter of famed Hollywood columnist Sidney Skolsky, who observed movie stars and other personalities from his perch at Schwab's Pharmacy on Sunset Boulevard and claimed to have been the first to call the Academy Award statuette "Oscar."
A graduate of Fairfax High School, Sidney-Splaver studied at the Actors Lab in Los Angeles. Her first movie role was in "The Eddie Cantor Story," a 1953 film her father produced.
Two years later, the dark-haired actress landed the role of Mil in "Rebel Without a Cause," Nicholas Ray's 1955 film about adolescent angst starring James Dean and Natalie Wood. Billed as Steffi Sidney, she played one of the girls in the gang of teenagers tormenting Jim Stark, played by Dean.
Decades later, teenagers still gushed about Sidney-Splaver's part in the classic film, she said in a 2000 interview with The Times.
"They just flip," she said. "I just find that amazing. They still identify with that movie."
A few more movie roles followed, including in "Hold Back Tomorrow" (1955) and "The Hot Angel" (1958). Then she left acting to write for teen magazines Datebook and Tiger Beat and work as a production assistant and associate TV producer. She also produced TV commercials.
After she married Splaver in 1985, they formed a public relations agency, Splaver Associates. They moved to Whidbey Island near Seattle in 1998, and she retired in 2003.
In addition to her husband, she is survived by her sister, Nina Marsh.
A memorial service is planned for 11 a.m. April 16 at Mount Sinai Hollywood Hills, 5950 Forest Lawn Drive, Los Angeles.