@Elfwine Poor word choice. My mistake. 
Totally not implying that she did the right thing or that a Guen who yelled at him is a bad person, though I guess I can see how my exact wording could be taken that way. This is the kind of thing I would catch in editing if I was writing a paper, but since this is an online forum I kind of just posted it up without checking it.
That said, I have to respectfully disagree with you that she would become angry/evil/embittered no matter what. I really do think that if this Guen talked to Arthur about it rather than hiding her real feelings behind a fake smile and pretending everythingās fine that they could work through it and have a stronger marriage because of it. After all, it was unintentional on Arthurās part (and Morganaās). Itās not like he purposefully slept with his sister. And even if it wasnāt, plenty of Real Life couples work through infidelity (though sleeping with your half-sister who also happens to be your wifeās best friend would certainly make marriage counseling MUCH more difficult).
As for her eventual vindictivenessā¦
(1) It will happen over a long period of time, not immediately after she sees the incident.
(2) I did mention in a prior post, there is something just slightly off about this Guen. Maybe not a sociopath, but something off enough that will cause/allow her to eventually act on her revenge fantasies instead of keeping them in her brain. That safeguard that most people have that says āBut I could never actually do that,ā that she just doesnāt have, or at least has one that is much weaker than it should be.

So many diverse opinions and ideasā¦so much to think aboutā¦
It may not be easy, but it will be possible.
(though I do know the answer, and I already have names and looks and personalities ā which will be different depending on biological father ā sorted out).