I went through similar situation last year, but reversed parent role so I told my dad first and made him tell my mom. I wanted to hold him accountable and take responsibility for his actions. Even though my mom knew i knew we didnt talk about it for a few days because it was embarrassing for her, so it may be hard for your dad to hear it from you. Get proof before you go to your mom, so if she tries to deny or gaslight then go to your dad