Being confronted such as that I believe Kurt's initial reaction would be reactionary offense. Maybe trick Zack somehow, get to mistakenly sleep with his sister. as retaliation. The end result being the siblings continuing the sexual relation much to Kurt's dismay... and Taylor being the older and still having unresolved feelings for Kurt chooses to convince Zack to create a situation where there is a lack of a way for Kurt retaliate further. That would could force him to turn his fury inward, and deal with his own demons.
Taylor's age and own interest add to the reason of why she would think of Kurt at all; And how he may react further and what would be best for him. Such thoughts would gift wrapt the realization that him manipulating a naive Zack was a failed attempt to humiliate her. Furthermore, I believe it would help the scene further if her reasoning to 'help' Kurt instead of 'hurt him is explained with pre-existing feelings for him...
As an added bonus this would add the category tag of Incest to duskford's quandary.
... As for Kurt, I've been thinking of using Zack's sister, Taylor, in order to get him to realize he's being a douche by having him fall head-over-heels for her, try to play his game on her, her turning it around on him, and then calling him out on it.
Reading a thread where Ashley convinces Zack that she is who he wants to be with, is what I have been daydreaming. Of course, the whole stealing a best friend's girl and other drama would just make it interesting. I have thought of writing it but am apprehensive about how duskford and others would react to it. Furthermore, I can be rather slow in developing threads further past their initial concept on my own.
In addition to the emotional turmoil involved. The way that came to my mind for Ashley to 'want' Zack is that he got her pregnant. That of course would be dialogue for 'after' the cabin; And, I am unsure if duskford wants people to go beyond the Cabin element. I mean, of course there would still be plenty of things said and drama at the cabin; But, that could be somewhat hinted at in flashbacks or just them referring to past conversations. Such as, it being difficult to 'forget' what someone says regardless of how much emotion is present.
The idea of how Ashley could know how Zack is the father is for genetic reasons. They can do prenatal genetic testing while unharming the fetus in any way as it grows. Say there was some genetic disease Brandon's family has and they now can eliminate while the child is in the womb. The specialist doctor could be a friend of Brandon's family and be obligated to tell him and from there Ashley's situation could help her decide... and add further drama. A genetic disease such as being predisposed to colon cancer. I believe most would choose to alter those particular genes if given the opportunity.
There are several possible paths from there. One more mischievous one being that Zack and Ashley pushed it past them as he went on to gain Victoria's interest. Being a month or two down the road, Zack wouldn't be the only one with misgivings with the idea of stealing their best friends lover.
Then my personal favorate of a category tag impregnation could be added to further frustrate duskford's dilema.
...
I like to think of my contributions as the "broken universe" where weird mixes occur, as Duskfrord said, where a normal conclusion of events don't necessarily happen, like Zack and Ashley's sudden sexual encounter. Unfortunately, I'm not sure how events can progress after a time...