WIBTA if I exposed the family secret after over 10 years to clear my moms name?

Very long story.. I apologize in advance. Also this is a throwaway account because I'm scared.

Family Background: My family and I (24 F) came to the US over 20 years ago. The only family we had here was a couple of my dads (55 M) siblings. Long story short my dad and my uncle (his brother) were very very close. My uncle has a step son (lets name him John) that is a little younger than my mom. My moms turning 50 soon for reference. John has a wife that's a little younger than him, we’ll call her Em. John and Em have kids together but they never got married.

John and his wife lived in an apartment above ours. Which meant that we spent ALMOST EVERY SINGLE DAY TOGETHER! Em was like my second mom at this point.

Now onto the good part: About 10 years ago, we noticed that Em, John and their family didnt want to hang out around as much as before. We never thought anything of it since they moved to another city neighboring ours. Within a couple months, Em cut us off completely. It was very weird and not at all like her. My mom still had no idea what actually happened or why Em and all of their mutual friends COMPLETELY cut off my mom. (This was a group of about 10 PTA moms that did EVERYTHING together) Fast forward 2 years, One of those moms (the only one that still talks to my mom) finally opened up to her. She said that Em started a rumor that my mom and John slept together and that's why she stopped talking to us. She told my mom about how every month when they have their PTA meeting this rumor had to be a topic every single time. My moms friend said she stood up to them and ruined her relationship with all of them because of it.

My mom swears on everything that never happened and I believe her. The irony of this situation is that there is proof that Em and my dad were having an affair. John has all the evidence and the only people that know about it are John, my mom and I. Of course Em and my dad know that we know as well. We've known about this affair for years but my mom never wanted it to come out because “it would ruin my dads reputation” but she doesn't care about hers?! Now everyone that knew Em and my mom ‘know’ that my mom is the cheater and the worst wife ever…. When in reality the only real cheaters were my dad and Em.

***My parents separated right after my mom found out about the affair, but this wasn't the only issue in their marriage. However John & Em are still together and actually got married about a year ago.***

My mom is prepared to take this family secret to the grave.. But I'm not! Would it be wrong of me to just expose Em and my dad after all this time? I kinda want everyone to see who Em really is since this isn't even the worst thing she's done.. but my mom says it's been too long at this point and I should leave it alone.