Answer: They went blind by getting there eyes pecked out.
Ashputtle is a story by the brothers Grimm. The adaptation, known as "Cinderella" is the more famous version nowadays. In the story, Ashputtle also loses a shoe and the prince identifies her by seeing how well it fits her. However, the two evil stepsisters meet with a different fate. On their way to Ashputtle's wedding, their eyes are pecked out by pigeons, leaving them permanently blind.