Also Katie doesn't know that you're fooling around with Emily.. (yet)
the first shock, which apparently did not “kill” Katie, took place recently - it was sleep-sucking further this was deepened during sleep-fucking&creampie(&possibly impregnating) - that is, Laurianne’s tummy will no longer cause Katie any surprise. she had already accepted it in advance. I think the process of the idea of sex with children taking root in Katya’s head has begun. Katie is unlikely to dare to run away from Max - for Max is the father of her daughter’s unborn child, the father of her future grandchild.
plus Emily was noticed nearby at the same moment - this was definitely deposited in Katie’s brain.
sooner or later, Katie herself will offer sex with Emily - Max need just give her enaught time. especially since Emily was not indignant or expressed disgust when she saw her parents having sex. the precedent of both daughters being found during their parents' sex has already been and is not deposited in Katie's memory in the form of a scandal (and where there is no scandal, there is tacit approval, and where there is approval, there is a hint of a desire to continue). it will work sooner or later as it should.
and this idea, sooner or later, will corrode Katie's protective bastion and she herself will be no better than Max (she become Max's Mistress) - she herself will be ready to invite Emily to join. or Emily herself will join when Max has sex with Laurianne.
although to be honest, it is not the reader who should tell the author about this - the reader should read this from the author under the guise of spoilers. haha =)
I think you, as an author, know more about the psychology of sexual relationships - otherwise you would not have written such a work. is not it?