Sam Sylk Show

Dear Sam, I saw my sisters husband in traffic with a woman. I honked my horn and they looked over at me, he pulled off. About 15 mins later he called me and said was that you I just saw? I let him know it was and asked who that woman was he said it was a coworker he was giving a ride. I asked if my sister knew he said yes. I don't believe him at all. It is starting to bother me. I want to say something to my sister but I don't want to get in her business and I'm sure he told her it was his coworker already. If it were you would you mention it? I want her to pay attention to the signs of him cheating if he is. Would you tell?