Hi, I'd like to help you with your question.
Although there is no way to tell for sure, it would not hurt to be more cautious around his wife's relatives. The problem here is finding a balance between being careful and not suspecting anyone of sexually inappropriate behavior just because they were abused themselves.
Has your brother spent a lot of time around his wife's family? Has he ever observed inappropriate behavior? Has his brother in law or in laws ever been with your niece's and/or nephew's alone? Were there any problems? How do these relatives relate to the kids? These are some of the questions to have your brother ask himself.
It also depends on the ages of the kids. If they are older, they may understand better than if they are just toddlers right now. Younger children will, of course, need watched much more closely. If his kids are older and able to understand, your brother can be sure to keep the lines of communication open. He doesn't need to tell them about the abuse, but he can be sure they know to come to him about any problems and he will listen and not judge. If the kids understand that, they will be more willing to express any concerns they have about odd or disturbing behavior they witness.
What your brother can do is bring up his concerns with his attorney. He can also ask his wife to not allow the kids to be left alone with these relatives. It may be inconvenient, but it is better than the alternative. He should try to explain his concerns in as nice a way as possible. He wants his wife's understanding and not more conflict. Your brother must also be respectful of his wife's family. They have not done anything to hurt anyone as far as he knows, so being cautious but respectful is the best way to handle it, until he has evidence otherwise.
I hope this has helped,