My brother-in-law is having an affair. What should I do?

I think it's best not to tell your sister for the time being, provided that you talk to your brother-in-law, discuss the problem clearly with him and see his attitude. Pay more attention to him and observe him when you have time. As a sister, you have the responsibility to protect your sister from harm. If he can turn over a new leaf, you can stop pursuing it, but if he has no intention of repentance, you must tell your sister.

Note: Tell your sister directly, and the problem will escalate. No woman can tolerate her husband's behavior, and your sister will be very sad, which will definitely have a bad influence on their feelings. Of course, you should do what you can.