cheaters make me sick

My stepdad just found out that my mom is cheating on him. He is the greatest guy she's ever been with, and it just breaks my heart to see him hurt. I've never been okay with cheaters, but I live with my mom and I know she will throw a tantrum if I say anything to her about it. I just do not want my child being around anything like this. I don't know what to do except cry.