How did Stheno and Euryale become like Medusa?
Stheno and Euryale didn't become like Medusa. Medusa became like
them. Before Medusa had become a woman with snakes as hair she was
mortal. Her sisters were not. There are two versions of Medusa's
story. One was that Medusa became cursed because she slept with
Poseidon in Athena's temple. The other story was that Medusa had
sworn celibacy to Athena, but forgot those vows and married
Poseidon.