At twenty-eight, Trina Elder's living a careful life, waiting for whatever is supposed to happen to happen. Only it's not. She's got a decent job writing copy for cooking products. Okay, so it's not the Great American Novel, but making turkey basters sound fascinating does pay the rent. So what that her older sister-the ''smart, successful one''-is happily married, and her younger sister-the ''really pretty one''- is taking on the world. At least Trina, the ''forgotten, Jan Brady one'' has her sensible Chicago apartment, her grown-up Garanimals wardrobe, and her cute boyfriend, Rick. Rick, who shares her addiction to E! True Hollywood Story, Rick, who's hot without being weird. Rick, who doesn't complicate things with marriage proposals.