SISTERS! Fans of Nashville Star -- USA's countrified take on American Idol -- know Tracy Gershon as the show's straight-shootin' judge. But who knew the Sony A&R bigwig is also rockin' actress Gina Gershon's older sis? ''She annoyed the heck out of me growing up, but now we're best friends,'' says Tracy, who advised Gina on the flick Prey for Rock & Roll and a concert tour (documented by IFC's Rocked With Gina Gershon). ''If I need to know someone in film and TV, she introduces me, and I do the same for her with music.''