I wonder what Myriam would do if she walked in on Marc and Katherine going at it? How many heads would explode if it were Anthony and Katherine? It's no longer incest, right?
Not that I think it will happen, but I do wonder about these things.
Anthony is definitively out of the picture, so Anthony and Katherine is not happening (though if it happened, for sure the incest patch would put it back as incest
;-)).
Marc and Katherine, strictly speaking could not be impossible.
Marc is still in, and by making Anthony and Myriam not married, is not even "step-brother" in the Patreon version, so it should be possible (then of course in the incest patch it would go to be brother and sister, complicating more life for Myriam, especially if
Myriam had herself "activity" with Marc).
Maybe Myriam only seeing it and talking about it later, rather than walking in, since "walking in", in the naughtiest case would allow also for the possibility of Myriam joining, or somehow instructing Katherine - there had been some past suggestions about that possibility.
But insofar as Katherine is there at the same time in which Myriam does anything sexual, Patreon could object - apparently Patreon had prohibited to Zorlun any sexual scene in which both Myriam and Katherine participate, even if they are not having sex with each other.
So, that would mean to stay in Patreon limits (at the border), probably the closest to a "walking in and joining" would be
Myriam coming in, sending Katherine away, and then seeing how she acts with Marc, and if he tries something with her (and then again, how she reacts to that).
And if while that happen (whatever happens, there is enough margin for the Myriam cuts the possibility, Myriam dom, Myriam sub in the way started with the "dream", or Myriam sub sub), would also have to be seen if Katherine really goes away, or stay around the corner and watch and hears what happens.
.
Notice if Katherine is in hetero or bi, at least as tendency, Zorlun indicated she is meant to have a soft spot for William (bi does not exclude men, and honestly, I know women who do not consider themselves lesbians, nor bi, but will do some sexual activity also with a close female friend, not only with men), and that would seem as going against her doing anything with Marc, although in reality it is not said 100%.
Notice also Zorlun had been explicit in excluding sexual activity with Katherine, since the only possibility originally foreseen was scenes in which Myriam would be the lead and maybe Katherine a kind of "extra", and given Patreon's prohibition.
I am one of those trying to pester him
a bit to allow some sexual content for Katherine (though considering Katherine is not Myriam, even if she is her daughter), and he did a concession in the last update, but it is to be seen how much he wil indulge some players in that direction, also because he is keen to keep the focus of the game on Myriam.
Plus, let's not forget there is Jennifer that has to play a role even with Katherine (she is close friend to her). Or maybe cousin Eve could also help Katherine with "tips"
and practical examples, and later even offer a massage to Katherine. I am not sure if Patreon's list included cousins, but I think it did not, Jennifer is not even a cousin, and as far as the activity is only with Jennifer and Eve, Patreon has not forbidden lesbian stuff.
Zorlun had also explicitly excluded anything between Marc and Katherine, and that, from before Patreon's intervention - so, even forgetting the other possible objections, indeed as you say "Not that I think it will happen"
, but...
There are many ideas that come to (my) mind about how things could go to reach a "Marc with Katherine" scene, but I decided it was better to cancel what I wrote as ISPOILER already - no point tossing too many brainstorming ideas around, they may end up complicating Zorlun's work, instead of helping him.