My wife's sister knows. She was the slutty one in her younger days, and liked teasing my wife about only having one cock (we were high school sweethearts that married young, and were eachother's first). Her sister was quite promiscuous before she settled down and got married. Eventually my wife told her, and now she's the envious one, because she married a conservative good ole' boy who would never share her.
7 of my closest friends know, 5 from first hand experience fucking my wife (most several times, only one was a one night stand due to his disappointing performance in bed), and two other friends that at least suspect, but aren't her type!
Her brother doesn't know that for nearly five years now his best friend (who he's known since childhood) has been fucking his sister, because my wife doesn't want to risk ruining their friendship or her relationship with him either. Her bother was very protective of his sisters, especially when one of his friends was sniffing around them, so we all agreed the less her brother knows, the better! My wife and lover feel a little guilty about that, but not enough to keep them from fucking eachother on a regular basis. In fact, they have a date scheduled for tonight!