The snake-sisters are Medusa's twins that are older than her and mistreat her because she isn't immortal like they are. They aren't the goddesses of anything, they just have immortal blood. Stheno, Euryale and Medusa once got told by Pheme that Athena's mother was a fly, so they mocked Athena and shamed her.
These snakes sisters' personalities match, as they are very in-your-face, arrogant, ignorant and they tend to tease people often. Although, Stheno is the meanest out of the snake sisters, Euryale is stupid and sympathetic, this was shown when Medusa died. They also like to shut people out and ignore everyone.
Stheno and Euryale are evil and unpopular students who dislike anyone who's not immortal like they are, however they treat goddessgirls and godboys fairly. Their personalities are very similar in the series and in the mythology.