Ladys i really need help right now.. Please read

Okay so I'm 21 and my parents that have been married for 21 years, together 23 is finally coming to an end.. They are separated now.. And are going to get a divorce in the near the future.. The problem I have is that my father is now seeing someone and I know and my mom doesn't really know and she telling me if she finds out it's going to be hell to pay. And it should be because their still technically married! And then my father made me meet the lady he seeing and I didn't want to and he's telling me not to say anything about this.. Like what the fuck!? Your still married to my mom so I feel like shit cause I want to tell my mom but I don't wanna go against my dad.. Im a female so of course I'm going to be mad he technically cheating on my mom even though they getting an divorce.. Am I wrong for keeping this a secret.. Is it my right to tell my mom.. I'm so hurt and lost right now I could die.. Please help me. Idk what to do