Sekai Meisaku Douwa: Hakuchou no Ouji (1977)

The Wild Swans

When a widower king loses his way in a dangerous forest, he is forced to take a witch's daughter as a second wife in order to escape. However, the newly crowned queen Greta quickly becomes jealous of the time the king spends with his children and vows her revenge. In the middle of the night, Greta sneaks to where the children are sleeping and turns six of the king's sons into swans, leaving the only daughter, Elisa, to fend for herself. Elisa travels far and wide in the search for her brothers and finds them living in a remote cave. While she rejoices at their reunion, she sees how terribly the curse has affected her brothers and promises to set them free at any cost. [Written by MAL Rewrite]