This is one of my favorite story types and includes a number of variants, including The Six Swans, The Twelve Wild Ducks, Udea and her Seven Brothers, The Wild Swans, The Seven Ravens, and The Magic Swan Geese. This particular version was told to the Grimm brothers by Julia and Charlotte Ramus and included in the first edition of their tales.
For the record, my favorite adaptation of this story is Juliet Marrilier’s Daughter of the Forest. In this version a wicked step-mother curses 6 brothers by turning them into swans and their sister, Sorcha, endures almost-unbearable hardship in order to break the curse. Be aware that this book does contain scenes of sexual violence. Having said that, it is a beautifully written story and I do recommend it. And now, on to our story!
It was then that the washerwoman told her that she had once had twelve brothers, but they had mysteriously gone away. Nobody knew where because the king had wanted to have them killed, and the twelve shirts belonged to the twelve brothers. The little sister was astonished that she had never heard of her twelve brothers, and during the afternoon as the clothes were drying and she was sitting in the meadow, she recalled the words of the washerwoman. After giving considerable thought to what she had heard, she stood up, took the twelve shirts, and went into the forest where her brothers were living.Grimm, Jacob; Grimm, Wilhelm. The Original Folk and Fairy Tales of the Brothers Grimm: The Complete First Edition (pp. 29-30). Princeton University Press. Kindle Edition.