A couple years ago I saw my brother cheat on his wife.
I believe that I'm the only one who knows about it.

No, I did not see him have sex with a woman other than my sister in law.
But I DID see him let a woman that him and I had just met that night
put her hand on his knee and run her hand up and down the inside of his thigh
which I'm sure his wife would consider cheating (the woman was sitting to his left, he was sitting to my left). He also gave her a peck on the lips right n front of me.

Even though it happened a couple years ago, I feel like his wife deserves to know about this. Part of me badly wants to tell her privately.

Should I just try to deal with the fact that I will have to keep this secret to myself for the rest of my life?