Well, any direct "interaction" betwen Myriam and Kath was killed by Patreon's threats, and (unfortunately
) Zorlun clearly indicated Cassius and the like are only for Myriam.
Zorlun had of his own indicated that Marc would not do anything with Katherine even before Patreon's intervention, and if I remember correctly, Patreon told him even "step-something" was off.
Maybe the fact in the revised version Myriam and Anthony were not married, would be enough to allow something between Marc and Katherine (and then, I could "incest-patch it" to brother and sister), but there is still the fact Zorlun had vetoed having Katherine in anything, to focus only and exclusively on Myriam (in other words, no "action" unless there is Myriam involved).
In theory, Zorlun has indicated that in his idea, if the player has chosen for Katherine to like boy, or be willing to explore both sides, she could have/develop a weakness for William (and Jennifer, I would guess, if she explores both sides), and if only girls, I guess that would go for Jennifer.
But maybe if there is enough request (hint hint hint ;-)), Zorlun will be willing to allow also something happening involving Katherine, even if Myriam can be only indirectly inspiring and not directly participating.
He is not going to completely change everything only because he got some requests, but on the other hand, he already had to make some big change to the story to satisfy Patreon's conditions...
"Indirectly inspiring", means e.g. hypotheses like
Katherine seeing or even just hearing Myriam "doing stuff" or even just getting in sexy costumes for the cosplay stuff, or finding the porn stash of Anthony or something made by Mark, and then getting inspired to try something.
Or viceversa, Myriam seeing Katherine doing or trying something or hearing about it or finding some proof of it, and talking about it with her (really only talking, given Patreon's stance) guiding her, a bit like that half-aborted conversation about the condoms.
Or if she has a weakness for William, and realises William has the hots for Myriam, trying to win him over.
Myriam herself commented that Katherine is not a little girl anymore, but in some things a bit "girlish", so, certainly she would do certain things in a different way from Myriam (though is visible she has the same sweetness and strong sense of trying to take care and protect others as Myriam), but it would be nice to have the possibility to develop that - and her friend Jennifer, who is probably more "easygoing", but also much more aware, in her approach to certain things, could certainly be an influence.
One good side of the game, is the fact one can somehow develop a bit the characters, by having choices about their behaviour and interactions, even with Myriam, Marc got that possibility and is an aspect IMO I like about him, it is a pity Katherine does not have it.
In the end, Myriam can influence how Marc develops, it would be only normal she also influences Katherine.
Well, just one opinion, anyway.
