Why is she doing this?

I just recently found out that my mums been cheating on my dad with my ex boss. She knows that I know, But she still hasnt stopped doing it. She quit her job and is starting a new job at his company starting Nov 1st. I dont know wat to do.. I dont want her to work there. How can I stop her from seeing him. My dad doesnt know anything about her affair. Its only me, my older brother and younger sista who know about this affair. And yet we still cant do anything to stop her from seeing him. Please advice