by Ashson
It maybe the wrong thing to do to screw one's sister-IN-LAW, but it is not taboo, and is certainly not Incest, which, by definition, involves close blood relatives.
I think it is taboo to screw your wife's sister. Accordingly, it is the right category! That being said, there are a few other categories into which it could fit!
As always, category correct or not, I always look forward to any of your stories.