You are right, except for the threesome, at least as many intend it (if you intend like the scene where she sees Mike and Myriam, then it is also possible to have a threesome).
The problem Zorlun had was that apparently Patreon extended and expanded the definition of incest, or at the very least uses the broadest one can imagine.
Even if there is no sexual interaction between Myriam and Katherine, the fact e.g. Myriam is "doing stuff" with someone, while in the same room Katherine is doing something with someone else, or something like Katherine kissing William while he does something with Myriam (or the reverse) would not be acceptable.
The simple fact one is present in the room while the other is doing sexual stuff and somehow participating to the sexual situation, for them is enough to get a big "red flag".
It seems the maximum extent they will allow is the already mentioned Katherine sees Myriam , or the one in the pool when they end up both without swimsuit in absence of any sexual interaction with anybody.
An idea launched, even by myself, was a triangular, but even if Zorlun does it, that would not be at the same time, so no threesome, i.e. either someone doing something with one of the two and then the other, or someone like Jennifer learning something with Myriam and then doing the same with Katherine