as for staying and being the father of the kids, he has made the right decision. since he cannot have kids, raising these kids will be the same as adoption like you said. so makes no difference. however, as for remaining married to the cheater wife, i personally wouldn't stay married. one, she cheated on him. even though she gave him two kids, there were resulted from her cheating. two, she did it twice, she will do it twice and another twice. and three, since you and your brother decided to conceal the truth from her, she will think that you guys don't know and will continue to cheat. i can adopt others' kids as my own and love as my own, but i cannot remain married to a cheater knowing she/he will most likely cheat on my again. these are some of the options, in my personal opinion, that he has:
1. he could either hide the truth forever and suffer the silent pain. if she cheats again and have another kid, he will have to continue being dumb.
2. tell her the truth and exposed her cheating ways so she knows that he knows. hopefully she will stop and hopefully they will live happily ever after. and do let her know that he is willing to love the kids as his own so she won't have doubts.
3. if after he exposes her and she decides to leave him, then oh well. what can he do. you can't make someone stay when she/he doesn't want to.
honestly, if he were my brother, i would tell him to confront her. i never want anyone to live a life of lies and suffer silently.