@Lucius Logan
There is a walkthrough up to including Part 8 somewhere in the thread, the new one is still not available.
@Siddington
Until now I had not the time to play the new Son routes, but I go by your experiences. As I wrote in one of my posts, Karen is really a piece of work, but what she tells the son in the evening according to your post is bull, no matter if she herself believes it. Why, please read further.
The cousin would like to have the Dad for herself (like her mom, the aunt) in some way, even if she does not really know why and if she is sober quite observant. Karen is the first to tell the Dad that he is lovestruck and has the hots for his daughter (in the last segment of the new stuff). If you are at least somewhat kind to her, she tells the MC about her plan that the daughter should have seen "her arrangement", not him.
If the Cousin is halfway as observant as gumdrop tries to paint her here, than she must have seen that the daughter is a total Daddy´s Girl and her relations with both Son and Mom are not the best, if more cordial than the Dad´s. And here is the point where the stuff Karen told the Son unravels. If the Daughter would have seen (or maybe even saw, we do not get to know it yet) Karen´s "arrangement", then yeah she might be appalled and angry for sure, but considering her relationship with her Dad she is more likely to think "those two cannot tell me anything anymore" and "I am the better choice for Dad". (look up social triangulation, Elektra Complex in parts and parentification, this is not so rare a path in consensual incest cases in real life, having studied social sciences comes in handy sometimes
)
In this update gumdrop paints a picture of Karen´s plans which do not really fit. On one hand she is observant enough to see that something is going in her cousins family, on the other hand she seems to miss that her cousin, the Daughter, has a wilder side and is very tight with her Dad. Then again, the way Karen tells the MC that his daughter will never give him "that", is more a denial to herself than him.
One thing is sure so, gumdrop for all his flaws, presses strongly on the drama tube. We´ll see what will come out of it. But personally I would like more stuff and development among the "main 4" than this much unneeded side drama.