The door opened, and Vicki stepped back inside to see Karen, her sister, still kneeling obediently in the center of the floor with her hands locked behind her head. Karen was naked except for a thin silver metal collar locked around her neck. Her body quaked as a vibrator, lodged deep in her sex, kept her off balance and on the verge of an orgasm. Their eyes met for an instant before Vicki looked away. In that instant bef...