Unwanted knowledge can be an onerous burden. If you know something that you really wish you didn't know, like your sister is cheating on your brother-in-law, it puts you in the untenable position of deciding whether you should do something about it. The good news is that you can stay out of it if you choose to. The bad news is that the knowledge will probably weigh heavily on you, anyway.
Get Your Facts Straight
Your first challenge is to figure out if what you think you know is true. If your sister came right out and told you she's cheating, you can probably take that to the bank. If you stumble upon a provocative scene, however, this is less clear-cut. Ask yourself if there's any chance that what you witnessed is actually innocent. Maybe it was nothing more than a joke shared between co-workers or friends. You have two options. You can clear your throat and interrupt the scene, gauging their reaction. This might be awkward, at best. Or, depending on your relationship with your sister, you can ask her about it later. You might be able to couch your suspicions with a "Did I really see what I think I saw?" attitude and a chuckle, and then let her explain – hopefully without taking offense. If your sister has a perfectly logical explanation, you're off the hook. Consider also the possibility that her husband knows what she's up to and that they have an arrangement. This, too, may be more than you need to know, but at least it's not your problem anymore.
The Bearer of Bad News
There's an old adage about killing the messenger, and it has a good basis in human nature. If you bypass your sister and go directly to your brother-in-law with the bad news, there's every chance that he's not going to thank you for it. More likely, he'll blame you for telling him, and he may even hate you for it. He'll also probably tell your sister that you told him. Now you've betrayed your sister, and you have two enemies you didn't have yesterday. Meanwhile, you have nothing to show for it all but a clear conscience.
Don’t Be the Black Sheep
Many families have black sheep, but do you really want to fill that role? If your sister and brother-in-law stop speaking to you, you probably have a lot of uncomfortable family gatherings in your future, with no good way to explain to everyone else what went wrong. If you tell them the truth, you've betrayed your sister yet again. If you don't – and families being families – others might assume that you're the one who did something wrong to create the rift. It goes without saying that every family member will have their own opinion about what you should have done and where you went wrong if you tell them what's going on.
Speak No Evil
Doing nothing may be anathema to you, but it might be your best bet. Ultimately, only you can decide. Your sister's marriage is going to sink or swim on its own – and that's as it should be. Consider how you'll feel if your own actions contribute to its demise. How will you deal with it if she ends up divorced, and she and her children are miserable as a result? If it makes you feel better to let her know that you know, that's fine. Offer some advice if you like, but then step aside and let her sort the situation out for herself. Tell her you'll be there if she needs a shoulder to lean on, and keep your eye on the big picture. Your relationship with her is forever. Her relationship with the other man probably is not.