You always reacted very strongly to incest stories, and especially to twincest stories. It's not exactly rocket science, but after seeing this in response to allusions of brother/sister incest three or four times I guessed... not necessarily that it was your weakest point, but that it was a weak point.
But I wouldn't use it on you unless it was a special occasion. When's your birthday?
Anyhoo, we should get back on topic before Roland brings out the big red hammer.