Should you tell your son that his wife is cheating?

yes you tell your sister that her husband is cheating blood is thicker than water you need to tell her as soon as possible No!!! Butt out. She will find out soon enough, and will not want to share it with anyone right away. Let her manage her own life. I would tell her only if you know for sure. If you have proof of any doubt. Because telling her and not sure, will only upset her and she will blame you for any fault to her marriage. Have proof before you act. But be prepared for what happens afterwards. If you have proof of his infidelity, why not you have a Word with your bro-in-law personally? You may help save a marriage if he comes clean with your sister after that. If you are absolutely positive and can prove it then yes, you should tell your sister, but don't be surprised if she reacts like you aren't telling the truth. The truth can hurt and it will take her a bit of time to digest this news or, she could already know and doesn't mind putting up with his antics. I wouldn't talk to this louse and give him a head start on priming himself for excuses he may use on your sister. Catch him with his pants down! Absolutely you MUST tell her, but you'd better have proof. Don't expect her to thank you either. Just give your information and tell her you're telling her because you love her. Then step out and let her deal with it in her own way and time. Absolutely, go to her with proof if you have it and break it to her gently. She deserves to have a husband that is honest with her and if he cant be tell her. She may already have an idea some- thing is wrong but doesnt think its that...