Would You Share This Secret?-OPP

Two sisters discovered something after their mother died. Now they can't agree whether to tell their brother or not about this secret that could shake up the entire family.

Mike & Kate-
My mother passed away just after Christmas. There are 3 of us grown children. My father passed away 2 years ago. My sister and I live here. My brother lives in Seattle. My sister and I have been getting my mom's affairs in order and getting her house cleared out and ready to sell.
While we were going through her things, we found a box of letters in a trunk in the back of her storage shed. They all had "return to sender" stamped on the envelopes.
Of course my sister and I read them. They were all for a man that apparently my mom had an affair with over 40 years ago. The affair lasted around a year and the letters started when my mom found out she was pregnant and from the letters it looks like the guy wanted nothing to do with her after that. We think he was married too. She sent 9 letters total, telling him that she was going to keep the baby and she still wanted to be with him. The last letter explained that if he wasn't going to acknowledge her or their baby, she was going to stay with my father and pass the baby off as his.
From the dates, my sister and I have figured out this baby must be my brother, the middle child in our family.
Now that we have this information, my sister thinks we need to share it with my brother. I completely disagree. She says he has a right to know. He never got along with our dad and maybe this will give him some peace as to why. I think that since he is 42 years old, why disturb the life he thinks he has? Not getting along with your dad is one thing. Finding out the dad you thought was yours is not, is a totally different thing.
My sister says she would agree with that, if none of us knew this secret. But how can we carry on and pretend like we don't know this?
We need advice. What would you do?

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