I had sex with my brother-in-law. What do I do?

About a fortnight ago we were on this vacation upstate (me, my wife, her sister and her sister's husband that is). My wife's sister's husband (I'll call him B) and I had met before and were close friends long before this trip. The third night in the two girls decided to hit the town, but neither me or B were interested in that so we stayed behind. We watched some TV, drank some beer and got to talking. The conversation got to a weird place eventually though, starting with some "What if" questions and confessions of mutual attraction. Somehow (I still don't know how) we convinced each other to have sex with one another. When it was over the gravity of the situation had hit us and we swore we'd keep it from the girls. They got home around sunrise and were none the wiser.

But while B seems to be doing okay I still feel just as sick over this as the day it happened. I want and don't want to tell her at the same time. Should I? And if I do will it make me feel even a little better?