Cut the bs you weren't going to stop what you were doing with your roommate that's why you're so furious at your friend for outing you, their help exposed your affair and brought honesty to your relationship for once. All the people like you who are screaming "Mind your own business!" like they always do in response to these kinds of questions are cheaters themselves which is why you're all so against someone telling in this situation, the guy who asked this question should tell it is his business since she's his sister in law and this effects his wife as well as possibly their marriage.