The Twin Dilemma (English, Paperback, Carolyn Keene)

When a star model disappears, Aunt Eloise insists that Nancy replace the model in a fashion show. Nancy reluctantly accepts the invitation, only to discover that several of the clothes for the show have been stolen! Once on the trail of her elusive enemies, Nancy discovers clue after clue pointing to a diabolical scheme that she must stop at all costs!