Medusa was the only mortal of the three sisters, and the only beautiful one. Poseidon raped her in one of Athena's temples. The other gods demanded that Medusa be punished for "defiling" the temple (never mind punishing Poseidon for raping her).
Athena changed her into a hideous creature with snakes for hair — interpretations vary on whether ...
None of the oldest sources seem to confirm or deny, since Medousa's sisters don't really do much in the most famous myths. But some later authors made it explicit.
From Nonnus's Dionysiaca, from the fourth or fifth century CE.
ἢ Περσέος εἶχες ἀγῶνα;
ἢ Σθεννοῦς ἴδες ὄμμα λιθώπιδος ἠὲ καὶ αὐτῆς
δύσμαχον Εὐρυάλης μυκώμενον ἀνθερεῶνα;
I did some research and the only poisonous snake in Greece which is also conveniently a viper is called the Ohia snake (Vipera ammodytes). Although when comparing that species to the 2 most famous statues of medusa's head (sculptors: Bernini and Cellini) the snakes do not match the Ohia. The head shape is similar but the scales on the heads of the snakes in ...