The Brothers Grimm collected this German fairy tale The Six Swans and it was also included a version in The Yellow Fairy Book by Andrew Lang.
The story tells how a king had six sons and a daughter from his first marriage. When his wife died he remarried but his second wife was the daughter of a witch. Although she was very beautiful her heart was evil. She hated her step sons and cast a spell on them that transformed them into swans. The brothers could only change into humans for fifteen minutes in the evenings.
Their sister resolved to free them but the only way it could be done was for her to make each brother a shirt made from starwort and for them to wear it. Furthermore, she must not speak, or laugh, for seven years, or the spell will never be broken and her brothers will stay forever swans. Dutifully she kept her silence and refrained from laughing and set about making her brothers a shirt of starwort each. Read more