Oh believe me I'm not suggesting that Karen's plan isn't flawed to all hell and back. She clearly isn't the sharpest knife in the drawer otherwise I doubt she would be in the state she often finds herself. She wants the Father either just sexually or for something more and she was desperately jealous of what she saw on the couch with F/D cuddling. She also has a general dislike of the Daughter due to her previous perceived belittling of Karen at school, as is discussed with the Son in his route.
In the conversation with the Son on his bed she is clearly thinking out loud when presented with the opportunity to instigate something. She uses it in the hopes that she can drive a wedge between F and D. Her plan hinges on the fact that she reasonably expects the Daughter to be repulsed by the idea of incest but as you said, she should reason that it mite not go down that way, especially in the light of her own incestuous feelings towards the Father. The Daughter is clearly enamored by her Father and it was fairly obvious to Karen. D mite have seen M/S naked in his bed and it blown away any reservations she had regarding pursuing her Father romantically. She may have thought "If they can do it, why am I holding back on my feelings?" Karen's plan was flawed but what should we expect from a flawed person.
@alex BukrI'm following the progress of the game Babysitter, and @Ignazzio just posted what the dev wrote on patreon, i'm posting it here because this is how developers should be.
Yes, that are definitely developers worthy of the label and something gumdrop will never reach, because his greed got the better of him.
@Siddington
We do not know at the moment how intelligent Karen potentially is, since she has two problems, alcohol and poor impulse control. Not for nothing some of Karen´s ideas would be called "Schnapsidee" in German, which means that these idea you have, looks good only if you are drunk as a skunk.
I definitely have to have a look at the Son routes, but yes, Karen is quite the schemer (the apple does not fall far from the tree considering her Mom)and she was extremely jealous when she saw D-D cuddling. Due to her poor impulse control, she started this new scheme with the Son. (By the way the Son is quite a highgrade a..)
As you said, Karen could reasonably calculate with the daughter being appalled seeing M-S naked on the bed (If she saw it, gumdrop did not spill in this update), but she should have seen early on that the next part might not go as she hoped for. But that is what you get if you let your impulses control you. Karen unintentionedly strenghtened the D-D relationship, because that may be just my gut feeling, but there are hints in the Dad route pointing towards that the daughter did see Karen´s "arrangement" too.