Wow so my dad is ( & was ) having an affair with a coworker and is now...?

Life's never easy, thats for sure. I think its ok to feel mad but you shouldnt feel his new gf is automatically a bad person. You should probably tell your father how your feeling. And you can refuse to stay with him for the summer, and you can also maintain loving relationships with your stepmother and your half-siblings. I think if you concentrate on the people and things that you love you will eventually get over this event. Just don't close your heart from anger, keep it open for your sake and your loved ones around you.