So the OM only told his wife so he would not be "beat" to it? This makes him even more questionable IMHO.
He knew you told your husband but kept calling???
What exactly is his motive here? He wants to keep seeing you wreck your life but not let his wife know anything, I don;t know sounds like a man who cheats as a hobby.
Are you positive he has not made a move on SIL? He may even have more oppurtunity there since they spend so much time around each other.
This is exactly why affairs are so bad look at the all the mess they cause and problems. I would not cut off family members either especially when you husband and his brother are so close. Many people are not close tih their family at all. But this does need to be resolved or else hard feeling will get worse and family gatherings will become uncomfortable then everyone will know.
