I know my sister-in-law is having an affair. What should I do?

If it's really like what you said, you should explain it to your brother according to the situation, but you should also pay attention to your way and make sure your brother handles it calmly. After all, if this matter gets out, it will affect your brother and sister-in-law and the whole family!

Whether your brother can accept this reality depends on what he thinks. If he can't accept this reality, neither can your sister-in-law. I think divorce may be good for your nephew, otherwise the disharmony between the two is even worse for the growth of the child.

Of course, it is impossible not to make mistakes. As long as your sister-in-law can change her mind and realize her mistake, you should persuade your brother to accept this reality for the sake of the children!

Don't say after finding out, as long as your sister-in-law doesn't change her current state of mind, your brother will find out sooner or later, and it may be more difficult to deal with then, and you will have a bad conscience.

So personally, I think you should explain it to your brother, but pay attention to the timing of joint construction!