There's the one section where Evelyn eats out Myriam during a spa foursome, but yeah, so much more was hinted at.
Evelyn has her own life. Unlike Myriam she works most days, and she's a very outgoing woman, with a lot of friends and lovers of her own. Her and Myriam though, have a close sisterly bond, which occasionally involves them getting naughty together. I expect there will be more sexual content involving both of them before the end of the game.
Evelyn could come home unexpectedly, and find Myriam in a compromising position with Marc or Cassius, and end up joining in. Then a week or so later, Myriam would come home and discover Marc/Cassius fucking Evelyn, and get a bit jealous.
Another option could be, if Myriam initially had sex with Cassius, but later put her foot down and stopped him. She could find out that he moved on to Evelyn, after they started living in her apartment. This would give the option for Myriam to have further content with Cassius, but with her in more dominant role, with Evelyn taking the submissive role.
Let's face it, we could come up with an infinite number of slutty scenarios for these two naughty girls, that will never see the light of day.