Apparently everyone takes turn with everyone, which seems to be the zetan lifestyle. I believe there is some gerarchy. Like i'm not sure if slaves are allowed to initiate intercourses among themselves if the owner doesn't give his benediction to that, but so far in Zaton's household i believe the only slave is Emily and maybe Fairy has a status akin to slave. Then Katherine is a concubine and the rest is family.
So far we have seen Zahra having fun with her daughters and the redhead minx (i don't remember the nephew name) trying to sleep with Ain. I mean, when Zaton wasn't directly included.