Cheater

As it's your brother in law who's cheating, I presume your sister is one the he's cheating on. If it was me, I would definitely tell her (I'd also then castrate him, but that's a different story).

I think you should put yourself in your sister's shoes, would you want to be told if it was your partner? Also, if she finds out about the affair, and discovers that you knew about it, how would she feel then?

I think at least, you should talk to your brother in law, and tell him that you intend to tell your sister.

It's a horrible situation you're in, I wish you the best of luck in dealing with it.

To tell someone that their husband/wife is cheating on them is not an easy task. If you know that your relationship with your sister is close then maybe you can tell her. Sometimes persons know what is happening in their lives and turn a blind eye but when someone point it out they become upset. Weight the pros and cons before you do anything.

We may not always want to get involved (who would really WANT to get inolved?) but I think it is important to let people know the truth. If my husband was fooling around on me, I'd want to know. I'd be upset if someone I knew was withholding that from me.

Stay cool a moment. It's better to let she found it out herself, then you may get in for the treatment. I had been there, never enter from the middle of this matter, your help will never been appreciated. Even more you'll get trapped between them, later you are the one who will be blamed, directly or indirectly.

Just keep your help for later. It's too fragile from here. You may try if you don't believe me.

i think just keep mum for sometimes. may be the situation can change in near future. who can say.also keep a tab on the brother in law. see if he is still cheating on your sis. if that continues, tell her.

This is a very difficult situation, when I was younger I told friends a few times when I spotted their boyfriends cheating and it wasn't an assumption - seeing a guy with his tongue down another girl's throat is definitely cheating but I ended up being the bad guy as on at least 2 separate occasions I ending up losing a friend cos the bf managed to talk the girl around and say that I was lying. There is an expression "Don't shoot the messenger!" but it doesn't really work. Since those days I have kept out of it when I have seen people misbehaving. I know a couple who I actually introduced about 18 years ago and they're still happily married - I used to see him out and about in bars and clubs chatting up other women and I told him to his face that he was a pig and I didn't introduce him to a good friend of mine just so he could behave like this. I stopped speaking with him for a few years and his wife (my friend) always asked why I was being so cold with him - I just said "you had better ask him, he knows why!" The funny thing is she gave him such a hard time about why I was not being friendly with him anymore that he stopped messing around with other women and now he's totally devoted to his wife! He told me that he had never actually got as far as cheating with other women but he would have had any of them actually taken his advances seriously but now he realised that if he had crossed the line his wife would never have given him another chance and he didn't want to risk that....

if im on your position, i will talk to my brother in law first and i'll give him a warning. but if he doesn't stop his affair i will talk to my sister and tell her that his husband is having an affair. i know it is really hard to be in that position. but of course i know you love your sister and won't take if your sister will be in misery.
pray for her too. have a nice day..