OMG!! TELL HER HUSBAND ASAP!! It is the best choice for this, and will and the least amount of anger and hurt in it, but, if you don't tell him, he may find out the hard way, and there will be even more tears shed, and nobody wants that. You will not make things worse by telling him, and it will get worse if you don't tell him....she may start sleeping with her new bf. I would talk to my sister (if I were you) and if she blows the roof finding out that you know, tell her husband. Tell your parents. If I found that out that my bro was cheating on his wife, I would confront him, tell his wife, and tell our parents. The more people on your side, the safer you are. If your sis gets ticked at you, just say that it was the best thing to do. Do the right thing, and tell someone quickly, not us at Yahoo! Answers.