How would Zahra becoming like the female protagonist of The Adventurous Couple would "explain the sluttiness of the daughters and the compliance to incest?"
They live in a dystopian world: the laws and values aren't the same as us. Sex in Zetan is seen from a different perspective, to the point where sharing one of your loved ones with a friend is seen as a form of respect.
Besides, why are you saying Ain and Shani act as sluts? Aside of being intimate with Zaton, Zahra, the household members and Igor (if you share Zahra and/or Ain with him), I haven't seen them act like that. They aren't going for any man or woman that appears in front of them.
And the incest thing... why should devs explain it? It's a kink vastly popular, they like it and a lot fans enjoy it too.