Hugh Hefner's Brother Keith Hefner Dead

Hugh Hefner's younger brother -- a fixture on 'Girls Next Door' -- died at his home in Beverly Hills ... TMZ has learned.

Sources tell us Keith Hefner passed Friday morning. He'd been battling cancer, but it's unclear if that's what caused his death.

He was 87.

Keith had been a longtime exec at Playboy -- handling Bunny training and recruitment (you're welcome). He also did some acting, appearing in a few movies and TV shows like the classic sitcom, "Get Smart."

He was married to model Caya Hefner, and is widely credited with inventing the Bunny Dip ... the sexy move servers used to deliver drinks at Playboy Clubs (you're welcome, again).