My dad is hardcore cheating on his girlfriend. What do I do?

My dad is dating his girlfriend for 2 and a half years, and she made me feel like I finally had a mom. For those who don't know, my mom remarried after coming out of the asylum because she became suicidal, why? My dad cheated and abused her. I was so happy that my dad was becoming finally loyal that I got this amazing bond with my almost stepmom, but for some strange reason he was getting mad all the time when he noticed I was getting attached to her as if she was my mother, and for a woman who lost her daughter from cancer, she finally got back that bond. Today she was on a business trip with her younger son, who's just started University, and my father was missing all day (as usually he was at work) but when he got back, he instantly started getting ready and told me that he'll be back soon. He arrived home at midnight while I was finishing a conversation on the phone, and I rushed to the kitchen because I haven't eaten anything yet, but he wouldn't let me pass because he said "I have a guest... so you better go back to your room.". I was a bit suspicious but I went back to my room with an empty stomach to read a book, twenty minutes later I could hear them having sex. And I could hear her voice, she was much younger than him or my stepmom, she must have been in her late twenties... I'm so grossed out I threw up. My dad hated my ex for cheating on me but what disgusted me more is that he is just like my ex... god I wanna vomit again. How do I deal with that crap?
Go and warn her
Tell my dad I know and give him a warning
Somehow find the whore and take revenge
Let it eat me inside
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My dad is hardcore cheating on his girlfriend. What do I do?
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