Please help! Need advice!!!

When I was younger I found out my step mom was cheating on my dad. My dad is my life we are closer than anyone can imagine. She came in my room asking me if these earrings were cute they were for so and sos daughter. I hacked her email. That so and sos daughter was the guy she was fucking behind my dads back. Broke my dads heart. He gave her another chance. Just thought she finally left it all behind last year and got close to her again. My little sister just told me she's cheating again. Proof and everything. Snap chatting dudes at 130 in the morning next to my dad while he's sleeping. Wtf do I do! This will break him. He might literally go kill these guys and end up in jail, again. He changed his whole life and is a general manager at a restaurant when he is an ex con. How can she do this, what do I do? I can't not tell my dad, I found proof going back to 2016. She must've never stopped!!!