Found out that my brother's wife cheated on him recently?

I found out that she has slept with my cousin a few times, and I believe it because he's one of those douche bags who likes to hook up with married women. My brother has been married to her for 7 years now and they have 3 kids. I'm really close to him and can't stand the fact that he doesn't know anything. I really want to tell him what's going on because knowing this is bothering me but at the same time I know that he will be hurt and he will fall apart.

Should I talk to him, or should I let it go because it's really none of my business (that will be so hard though). If I talk to him, how should I go about it?
Found out that my brother's wife cheated on him recently?
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