Right! Here's a different solution to tangle of mother and sister shit rendering The Genesis Order incest free.
(1) Heather was William's father's girlfriend who lived with him but not William's (biological) mother or step-mother by marriage;
(2) Chloe was fathered before William's father and Heather met and is not William's (biological) sister or step-sister by marriage;
(Basically same as before but with marriage omitted and so both consanguinity and affinity no longer considerations.)
Under these circumstances William, Heather and Chloe are a genuine (Jerry Springer) family of sorts, which explains their closeness, but William is unrelated biologically to both Heather and Chloe and, because William's father and Heather never married, also also not related to Heather and Chloe by affinity opening up the possibility of intimacy between William and both Heather and Chloe, individually and separately, without such intimacy construed as incest although it would be kind of distasteful, unsavory and weird all things considered I have to admit.
Can we drop all that whimpering, bleating and hand-wringing about the lack of incestuous content now?
(Because technically there is none with this scenario.)
Ta-da.